I can forgive someone for failing, but if they just let me down in a way that shows they weren't capable of doing something and didn't tell me that. Like, if someone always flakes on me when we have plans it's over. They could have had the decency to tell me they couldn't make it or put in more effort to keep to the plan. If they miss ever once in a while no big deal, or they let me know they can't or aren't sure, but to consistently let me down with intent to correct breaks my trust. In similar way, if I ask someone for help and they just never help me even though I have helped them that's it, we're done. Not always being able to help is fine, but never being able to, or even putting in any effort or trying to fund someone who can, means they don't value me as a friend. Worth noting most of this applies to closer friends, not casual friends. I don't put a lot of trust into casual friends and don't expect it from them. I do also always put in effort to reach out to my friends and be there when they need me, check in with them and make sure I'm being a good friend to them. So once they've crossed the line they don't get that anymore. I can forgive and let go my anger, but I can't ever trust them again. That might sound harsh and lonely to you, but I have been betrayed before and I can't let it happen again. Besides, most of my friends have no issue not violating my trust.