Liz.....at the risk of sounding "textbook"....I will give this a stab.
I suspect that extreme people pleasing has it's roots in early childhood development....when basic feelings of self worth and self esteem establish roots.
especially, if one has had a chaotic childhood or abuse (especially sexual abuse).
One might have to dig deep into the Family of Origin stuff, in order to get to the heart of it. Of course, that requires a special professional, trained in this area...not just any ole counselor or therapist....Often, there is a deep shame..and a feeling of not being "good enough", at the core....That can take some deep digging to get to...
Also, someone cannot just snap a finger and have good "self esteem"....one has to have esteem building experiences as an adult...just like a kid does....
It is through experience...not just intellectual awareness, alone, that one builds self confidence and self esteem....
What does it look like /feel like?
this is what I think....When you can do what feels natural without anxiety about it....(most of the time, anyway)...
And, when you can walk into a room of strangers and the first question that you ask yourself is: "Do I like them"....not..."Do they like me"......
***by the way...you do not have to like..every...body....That isn't natural...
You have only been away from the relationships that damaged your self for a short while...compared to the decades that you marinated in them.....