I have two understandings that may help in this regard - the negation of existence/nonexistence and the affirmation of dependent arising. I will substitute "form" for "exist" just to make it simpler. Additionally, I am still learning so please feel free to object to any points.
In terms of "exist", it is easy to see that nothing truly exists on its own. Everything is made up of parts which are, in turn, made of parts. A table, for example, never truly exists. There are trees, bits of metal perhaps rubber or plastic, and other materials that go into assembling what we call "table". We typically don't look at objects this way and it takes a little bit of concentration to do so. Those parts are also made of parts all the way down to subatomic particles.
Beyond even that, things come to be only from causes. The parts cause the table, and other causes cause the parts themselves. There is no start to this cycle - everything that comes to be is dependent on its own causes.
Thus, if everything comes from causes, then nothing exists inherently and is temporary. Thus, if something were to exist in a permanent, inherent way, then it would have to be uncaused. Additionally, it could not change as that would indicate cause and thus violate any sense of permanent inherent existence.
We don't see things this way typically. We think of "I" and "you" and "this" and "that" as if things are substantial in an inherent way. Even "I" am not the same physically or mentally. The atoms making up our body turn over constantly, and our mind is simply a series of mental moments, one causing the next moment to moment. Thus, the "me" I know is not the "me" of ten years ago or even one week ago.
When it comes to "non-existence" in light of this, then it is easy to see that such a concept is nonsensical. You would have to presuppose the possibility of something existing (in an inherent, causeless way) in order for it not to exist. Since we have already seen that permanent, inherent existence is a conundrum, likewise any notion of non-existence is as well.
So we are left with the dilemma - nothing is in a state of existence or non-existence. How do we solve this?
The answer is known as dependent arising. Things "exist" but not in a way that is permanent, eternal, or inherent. In other words, things only "exist" in a relative way. Everything is caused by the immediate moment before and, in turn, becomes the cause of the next moment.
Thus, we experience reality as existing on a relative level. It is truly there as "something" that comes into awareness from the immediate moment before and passes away as a cause to the next moment. We never experience anything in a permanent, self-inherent way as that is nonsensical. At the heart of what we perceive as reality is therefore emptiness - that is, everything is lacking inherent existence. There is no such thing.
Thus, you are in form in a relative way as are your thoughts and feelings. They do "exist" in a relative way. But they are all dependent on causes and do not arrive into awareness out of nothing (i.e. self-created inherent causeless existence).
Source: Nagarjuna's Mulamadhyamakakarika https://www.amazon.com/Nagarjunas-Middle-Way-Mulamadhyamakakarika-Classics/dp/1614290504