In a way it is like being stuck in the continuous present tense. You can't relax and just enjoy a situation, you are always on guard and reading others for cues of either harm or rejection, and ultimately abandonment. It is emotionally and mentally exhausting, and also to feel like you have to take on others' emotions, as you say - like how you start feeling the same way. (For me, this isn't even a conscious choice). Then you lose touch with your own emotions, because you are feeling what the other person is feeling so its destabilizing at times, and probably confusing. I don't know if you struggle with boundaries at all, but I do. I feel they are non-existant at times, and like I'm not even allowed a boundary. I grew up in an enmeshment type situation though.

And I can relate to having a positive encounter with someone one day, then a negative one the next - and all you can remember is the negative one then. The positive feelings don't sustain or carry over. It's like the last encounter is frozen or burned into the emotional memory bank and it becomes the factual one. Makes relationships very challenging because you don't know if you are going to be in a hurtful situation with the person. It makes everything unsure because you can't sustain the loving and positive interactions over time to add up to a secure attachment with a person.

I've also heard how those with BPD (or perhaps it was depression or something similar) read negative connotations or meanings into neutral faces or expressions. It's like you expect others to be hostile at some point, or at least negative and hurtful and rejecting towards you. So then it is very difficult to be in relationships because it is very difficult to trust the other person.