Wondered if anyone has any tips on responding assertively in the moment? 

If someone is actively hurtful, I find it far easier, but there's so many subtle ways people can be invalidating, condescending or trigger anxiety. When it happens my mind tends to space out as they continue talking and if I confront them, it's nearly always afterwards, which can feel completely pointless if it's something small.

I'm caught between struggling to read people during my interactions with them and distrusting my memory of said interaction because of my tendency to react to what was said later.

I know this is most likely a trauma response on my part and a way of people pleasing, but it's incredibly frustrating.