I'm getting back to the first question about my problems: when I am reacting in a way that looks like overreacting, is it because I'm overreacting or because someone is abusing me? Hey this is such an important question!
In my life I've gone through phases of thinking it's all my fault and thinking that it's all their fault. Definition of borderline says it's all her fault, the definition of a codependent says they tend to get involved with people with whom they become a victim of psychological manipulation. From this point of view is clear that depending on where we put the blame, we get different diagnosis and different model of the situation! For the last 7 years I've been using the borderline model, and it got me into an abusive relationship. This model was bound to put me into trouble as it doesn't assume that a borderline should learn to protect themselves from toxic people, as it puts all the blame on her. But I see the risk of flipping into the opposite side and always suspecting manipulation or provocation, as I know for sure that sometimes I overreact because of how I am, not what others do (I can overreact at the weather, that's a sure counterexample of a manipulative abuser).
So I came up with a work in progress explanation:
Everyone has certain triggers that make them react stronger in a negative way. And I'm not talking only about the visible reaction, for example I am sometimes made felt really shitty by some innocent remark of someone, but usually I don't act in any overt way on it. So under the trigger is an emotional wound that can be triggered in certain situations. And then there are people who scan for those wounds and triggers and use the latter as tools. So when I'm triggered the first time - my fault, projection, it may remind me a situation from the past, simply overreaction. But if it happens in a consistent way with someone regularly - it's being psychologically abused. Someone is just mining the fact that I can be triggered in that way.
And now, in order to check if with person A wound B is triggered regularly I have to be able to distinguish wound B from wound C. As just knowing that a person A is triggering me doesn't yet prove the intentionality on their side. It has to be a repeatable set of wounds that are triggered. Even the triggers can vary. That's why it's important to keep a mental catalogue of wounds and when a certain person triggered a certain one. And a prerequisite to it is knowing your wounds. Not just triggers or the impact of them. To look at an emotional wound as a whole: where it originated from, which feelings it causes, which situations trigger it. Only then can one get some clarity. Because hell yes, I can work on not overreacting even if I'm triggered, but when I'm dealing with an emotional manipulator they'll make sure to keep me busy overreacting. That's their tactic.