How do you decide which people are worth fighting for?

I usually leave people when they start affecting mental health which is after i give them a few chances (maybe more than a few). Recently I feel the urge to leave if I get the slightest bit of discomfort or they affect my peace a tiny bit but if I do that won't I eventually have no one? Sometimes you fight for people and it breaks you in return because you thought they were worth it and I don't want to make that mistake again.

How do you decide which people are worth fighting for?
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