people pleasing/chameleon behaviour

I don't make friends or have close relationships much and not at all now, but when I was younger I got kind of adopted into groups every now and then or people would gravitate to me. What would always happen is that I turned into a chameleon and started adopting their ways of speaking, mannerisms, interests etc. to the point where I would lose my sense of self totally. I would probably also pick up on their likes and dislikes, buy them ridiculously expensive presents for christmas and birthdays, be mortified if they caught me in a mistake. I was never trying to act, this all happened automatically, and I would kind of think it was a good thing I was doing until it would suddenly get tiring or I would start to see things about that person that I didn't like and then I would just quite suddenly move away from them and stop hanging around with them. Because I would exhaust myself or just feel depressed because they thought they knew me but they didn't really and the real me and my real life was maybe starting to cause me issues.

I mean I've cut people out for all sorts of reasons before but this type of exhaustion used to happen periodically and I wondered if other people had experienced it because it's not something I see associated with STPD, I guess because it's assumed that anyone who has it just doesn't ever get near anyone. And it's a "co-dependent" kind of behaviour.

Last time it happened was with someone I think might have had NPD or BPD or something like that, he definitely had anger issues and bullying tactics, and I got out of that pretty quickly but obviously without awareness people pleasing can lead to abusive situations with people. So I guess I'm also interested in how easily abused someone with STPD might be or if that's a thing people have experienced. Only if you're comfortable sharing of course.

Any thoughts on this?

It's associated with borderline personality structures.
Basically it's due to poor boundaries, and an unstable sense of self.

I did that a bit in the past when I was younger, not so much anymore.
It's useful to get into places you don't belong in.
This being said, I never lost my independence, it's just I might have likes some features others had so I adopted it. I still might do it if it happened, I just never saw the need in quite awhile.