Buddhism has made me realise that everything is impermanent and undergoing the process of destruction including intimate relationships however if I know this then why is it still so painful when it happens? And what is the point of trying to build a life together with another person when it's inevitably going to end? Sometimes it all feels like such a cruel joke. I was in a relationship for 15 years and never thought it would end but it did and 6 months later I still feel so sad. I don't want to ever get involved intimately with another person ever again because I don't ever want to go through that pain again. Yes this may be aversion to pain but why put yourself through that if you can avoid it? Sure there will be more pain from other things but the pain of separation from a loved one feels worse than a death. It actually feels like I could die.
Ajahn Chah once explained well how to have a relation, here in a simile of a glass:
You may say, "Don't break my glass!" But you can't prevent something breakable from breaking. If it doesn't break now, it'll break later on. If you don't break it, someone else will. If someone else doesn't break it, one of the chickens will! The Buddha says to accept this. He penetrated all the way to seeing that this glass is already broken. This glass that isn't broken, he has us know as already broken. Whenever you pick up the glass, put water in it, drink from it, and put it down, he tells you to see that it's already broken. Understand? The Buddha's understanding was like this. He saw the broken glass in the unbroken one. Whenever its conditions run out, it'll break. Develop this attitude. Use the glass; look after it. Then one day it slips out of your hand: "Smash!" No problem. Why no problem? Because you saw it as broken before it broke. See?
But usually people say, "I've taken such good care of this glass. Don't ever let it break." Later on the dog breaks it, and you hate the dog. If your child breaks it, you hate him, too. You hate whoever breaks it — because you've dammed yourself up so that the water can't flow. You've made a dam without a spillway. The only thing the dam can do is burst, right? When you make a dam, you have to make a spillway, too. When the water rises up to a certain level, it can flow off safely to the side. When it's full to the brim, it can flow out the spillway. You need to have a spillway like this. Seeing inconstancy is the Buddha's spillway. When you see things this way, you can be at peace. That's the practice of the Dhamma.
That's the case how to possible think if having or losing a relation.
The other case is the sub-question:
And what is the point of trying to build a life together with another person when it's inevitably going to end? Sometimes it all feels like such a cruel joke.
Realization that becoming is actually a "curel joke" is a very high realization and if seen in all compound things the reason for earnest seeking a path out, blessed if having come to the Buddhas good teachings. This is meeting up with the reality of Dukkha.
So in regard of search, what will be for a long time benefit? That search it self is bound to much suffering as well, is clear, but if not having a search is needed and there are three kinds:
This was said by the Blessed One, said by the Arahant, so I have heard: "There are these three searches. Which three? The search for sensuality, the search for becoming, the search for a holy life. These are the three searches."
the Awakened One's
how searches come into play,
where they cease,
& the path to their ending.
With the ending of searches, a monk
free of want
is totally unbound.
Search for a partner is nothing but about searching after sensuallity, maybe becoming, isn't it? Of course after a career even more... so just give it a deeper thought and maybe use you luck of independency you currently have for a more holly life to be.
At the End it's maybe worthy to say, that also searching for a relation to be able to live the holly life is actually nessesary, so admirable friend(s) are always worthy to seach for and also to invest much in such a relationship, even of course it will outardly break, but once being part of the other kind, no and never alone and without support till standing firm alone.
(Note: This is a gift of Dhamma and not meant for commerzial purpose or other worldily gains)
the Buddha actually called this painful feeling "death" ('marana') although most Buddhists believe the Buddha was referring to physical death
Now delight in feelings is clinging. With his clinging as condition, becoming comes to be; with becoming as condition, birth; with birth as condition, ageing and death, sorrow, lamentation, pain, grief and despair come to be. Such is the origin of this whole mass of suffering. MN 38
15 years is a long time, in which the other person became part of your psyche. This becoming is called "becoming" ('bhava'). The dual psyche is called "birth" ('jati'). Now that dual birth psyche has died & it takes time for the psyche to restore itself to singularness.
As for relationships, Buddhism provides guidance however, in the conditioned realm, nothing is certain:
Buddhist Impermanence is best understood in the context of the Three Marks of Existence: Annica (impermanence), Dukkha (unsatisfactoriness), Anatta (non-self).
Trying to understand it on its own will lead to more suffering like you are going through.
Anatta is close to Śūnyatā (emptiness) - though context can give it slightly different meanings if we are pedantic.
By empty or non-self we mean anything you can name (have a sign for) is made up of things that are not it. A flower is made up entirely of non-flower elements such as sun and soil and seed. When the elements of the universe are arranged in a certain way we say a flower has manifested but if we look deeply it is difficult to say when it truly started and when it stopped. The conditions were always present. The flower continues as our joy at having held it and as the compost nourishing other flowers. There is no little flower soul that is snuffed out.
Looking at impermanence from the perspective of non-self the flower is impermanent in that it never had a separate self and never really ceased to exist. We just had a sign for it for a while.
So when you say things are "undergoing the process of destruction" that may be a wrong view. Things are in constant transformation. No separate thing is ever created or can be destroyed.
A little girl said to my teacher (Thich Nhat Hanh) that she liked impermanence because without it she could never grow up. But impermanence means she will grow old and die. Looking deeply we can have joy for her whole life even when we know it will end in death.
It is the same for your relationship. You are part of its continuation. You are walking, sitting and breathing as its continuation. By taking care of your current suffering you can transform it into something beautiful. You do this by seeing that your suffering is impermanent and empty (entirely made up of non-suffering elements i.e. made of the joy of your relationship). It may take time.
The way I look at the three marks is that embracing non-self & impermanence leads to nirvana whilst any desire to deny them leads to suffering. It is not impermanence that causes suffering but our denial of it. (see The Heart of the Buddha's Teachings by Thich Nhat Hanh)
If we can really see impermanence and non-self then every moment is not only precious but includes the entire cosmos - which is pretty special.
Everything that has a start will eventually have an end. Even in relationships. Even relationships that last a lifetime come to and end with death. If one is attached to such relationship this also is unsatisfactory when it ends.
Also you are not entirely in control of a relationship. Hence what one is not in control leads to unsatisfactoriness.
As for lay people it the lay code of morality is the 5 Precepts. Though any relationships is not entirely satisfactory as long as this is not sexual misconduct it is OK, keeping in mind this also will not be entirely satisfactory and more the attachment more it will hurt when it ends.
Also you can build temporary understanding Brahmavihara hence lessen the unsatisfactoriness.
To get over unsatisfactoriness completely you have to look beyond relationships and aim for Nirvana.
For a start or for context, this topic says something about how to choose a partner: Any authentic sutta from any tradition that gives guidance on what kind of partner to choose?
You're asking two questions: why have a relationship, and how? I'm not sure that "why" is the best question. It seems to me that there are people around, you have relationships (historical, geographic, social, economic) with them, and the question is what relationship to have and how, rather than why. Maybe it's worth considering "why" (i.e. your intent) but only in the context of also considering "what". There are also bits in the Pali canon which say that it's better to have no relationship than a bad one.
Buddhism has things to say about (it seems to recommend) admirable friendship, though that may not fit your question. It's worth mentioning that lay people may be expected to have several "relationships" of different types, e.g. explained in the sutta about the six directions: Sigalovada Sutta (DN 31).
Lastly you mentioned that you "never thought it would end". For a start my experience is that there's both less time and more time than you expect: "less time" in that it's already finished when you think it's continuing, and "more time" in that something remains present after you expected it would be over.
More importantly though, "thinking it would end" might actually be a helpful or a beneficial thought, and "not thinking it would end" detrimental to a relationship. For example, from the Dhammapada:
Verse 6: People, other than the wise, do not realize, "We in this world must all die," (and, not realizing it, continue their quarrels). The wise realize it and thereby their quarrels cease.
- There are those who do not realize that one day we all must die. But those who do realize this settle their quarrels.
I think that when you know you're dying, for example, or when you know your partner is dying, you may become better: more generous, for example, better at making peace perhaps. So although "never thinking it would end" does sound romantic, sometimes it can be more unwise than it is admirable.
Buddhism has made me realise that everything is impermanent and undergoing the process of destruction including intimate relationships however if I know this then why is it still so painful when it happens?
Well done, you have realised intellectually that things are impermanent but you are still attached. Intellectual knowledge doesn't end attachment (but it can help). To end attachment we need real knowledge, with real knowledge you will instinctively know that things are impermanent.
I think you have switched from attachment to having a relationship to attachment to not having a relationship. Whether you have or don't have a relationship is not the point, the point is how we think when do/don't have a relationship.
While you are in a relationship there is some pleasure from love. You can experience this pleasure with wisdom and work towards your relationship with wisdom. Doing it with wisdom means that you know that it is impermanent so you can do it without attachment.
Don't forget that you can be attached to single life too and that also causes dukkha. You think that if you're single you can control your relationship status and make yourself constantly single, but this is also impermanent and can't be controlled so it is a source of dukkha.
Real knowledge is quite like an instinct, it is effortless. With intellectual knowledge we have to remember what we learned and then use thinking apply that to the situation, it's slow and takes a lot of effort.
For example, if you know intellectually that things are impermanent then if you drop a glass and it breaks your initial reaction will be "Damn I liked that glass, now I've lost it", you are suffering at this point but then you remember "Everything changes, the glass had to break so I never could control it". This thinking might reassure you and calm you down. If you have real knowledge of impermanence like an enlightened person then before the glass breaks you are not viewing the glass as something which inherently exists, it's a temporary phenomena. When the glass breaks it has no effect on you because you never thought of the glass as being not-broken.