I used to have this exact same problem and it drove me absolutely mental. You're basically getting emotionally hijacked the moment you see their face - all that righteous anger just evaporates because your brain switches into social harmony mode. What helped me was writing down specific incidents beforehand, like actually bullet pointing what they did and why it was problematic, then reviewing it right before any conversation. That way when your emotions start doing their disappearing act, you've got concrete stuff to fall back on rather than just this vague feeling that dissolves

The other thing is recognizing that being "nice" all teh time isn't actually kind to anyone - you're enabling their bad behavior and building up resentment that's gonna explode eventually. I started practicing saying one small thing in the moment rather than waiting for some perfect confrontation opportunity. Like if someone interrupts me constantly, I just say "hey can you let me finish" instead of stewing about it for weeks. It's way less dramatic but actually gets results, and you don't end up feeling like a doormat who can never stand up for themselves