Does anyone have a useful visual metaphor or story around this? I try to see myself as a rock and their words like arrows pinging off me but it doesn't work.
I once read the opposite metaphor.
Someone, perhaps a yogi, magically made their body hard like a diamond -- and fought with someone else, trying to prove that they (with their diamond-like body) were superior. But the other person was even more invulnerable (therefore superior) -- because weapons just passed through his body without resistance, as if the body were smoke, insubstantial.
I think that's a metaphor about "ego" or something -- "identifying". It turns out though that the story is from a non-Buddhist source, I forget which one. And I'm not sure the strategy is sustainable.
A Buddhist metaphor I like is Akkosa Sutta: Insult (SN 7.2) -- which is "magnificent".
Incidentally that's inline with instruction from my martial arts teacher -- who said that if someone from outside the class offered to spar (or fight) then we must decline or refuse their invitation (to participate).
Specifically when someone you love or whose opinion matters to you greatly attacks your way of life, your beliefs
I find it unfortunate that "great attacks" should ever happen, or escalate to that.
The sutta Tittha Sutta: Sectarians (Ud 6.4) says that sectarians wound (or try to wound) each other with "weapons of the mouth" (i.e. "harsh words" I presume) -- and says they might argue because they see or focus on different aspects of something -- "my view is right" ... "no my view is right" etc.
Sectarian argument is also attributed to "conceit" -- more-or-less defined here or here.
... or even on a smaller scale just makes you feel ignored, pushed around, etc.
I don't know, is that the same thing? Can be related -- but isn't always?
There are a couple of Zen stories about feeling ignored and being pushed around -- the idea of feeling ignored reminds me of The Taste of Banzo's Sword -- and pushed around, Obedience.
I guess my point is that feeling ignored (not given what you want), and being pushed around, might be to some extent the way things are in adult society -- something which the ego may object to -- and something which, to some extent, you can train yourself accept.
Perhaps it's a matter of degree though.
I think my other point is that "attacking" someone seems to me fairly questionable, but "ignoring" someone might be not so blameworthy.
I've felt ignored and maybe disliked it too sometimes, so condolences. But perhaps this lack or desire is something you might fill from within somehow, instead of requiring someone else to supply it.
One of the themes of Buddhist (monastic) literature is that it's ensnaring (not freeing) to maintain lay social relationships -- Khaggavisana Sutta: A Rhinoceros (SN 1.3). Some say that's an old sutta, pre-dating later practice of Buddhist monks living together in institutional monasteries. You might reach the same conclusion (that it's better to "go forth"), even if you don't you might it insightful and useful.
I find I quickly lose confidence in my "inner voice" when my loved ones, particularly family, do this. Generally when a stranger does this to me it's easier to let go.
I'm not sure what you imply by "family".
There a sutta Sigalovada Sutta (DN 31) which describes different types of family responsibility -- and mentions that how you treat your wife or husband isn't the same as how you treat your parents or children.
I kind of like this description of an ideal partner.
It's not easy to address your specific situation since you give so little detail. And generally I think that a relationship therapist might want to talk with more than one person in the relationship.
How to develop fortitude?
Well. In the abstract I suppose "fortitude" means "courage".
You might like to investigate what Buddhism says about each of the Bodhipakkhiyādhammā or possibly the Pāramitā -- conviction (faith), virtue, skilful virtue, wisdom, effort, patience, generosity -- and Brahmavihara.
Wikipedia defines virya for example as,
Vīrya (Sanskrit; Pāli: viriya) is a Buddhist term commonly translated as "energy", "diligence", "enthusiasm", or "effort". It can be defined as an attitude of gladly engaging in wholesome activities, and it functions to cause one to accomplish wholesome or virtuous actions.
Another sutta says that "skilful virtue" (which I guess means, doing what you know to be right, not doing what you know to be wrong) should result in a lack of remorse -- and it seems to me that perhaps "remorse" might be the opposite of courage or enthusiasm.
One other thing, in deciding what's right -- the Kalama Sutta (AN 3.65) includes,
Of course you are uncertain, Kalamas. Of course you are in doubt. When there are reasons for doubt, uncertainty is born. So in this case, Kalamas, don't go by reports, by legends, by traditions, by scripture, by logical conjecture, by inference, by analogies, by agreement through pondering views, by probability, or by the thought, 'This contemplative is our teacher.' When you know for yourselves that, 'These qualities are unskillful; these qualities are blameworthy; these qualities are criticized by the wise; these qualities, when adopted & carried out, lead to harm & to suffering' — then you should abandon them.
... as if you should believe your own opinion, what you "know for yourself". But it also talks about what praised or criticised by "the wise", as if you should take their perspective into account and non only your own.
Buddhism is a bit like that, neither one extreme ("Do whatever the hell you feel like"), nor the other ("Do whatever everybody tells you"). Or sometimes the advice is situational, "Be a lamp unto yourself" in one context, "The holy (monastic) life is living with a spiritual friend (i.e. with the Buddha)" in another.