Do people change?

I’m still going through the motions of a breakup and my ex was quite troubled. In all fairness to her she definitely had some undiagnosed mental health problems due to an abundance of trauma from childhood (e.g. grew up poor, not a good family life, no experience in our field so she’s going to have a hard time landing a job). She just wasn’t a very good person in general and was constantly negative and draining my (and my friends) energy. She just wasn’t a pleasant person to be around.

I do find myself missing her (I don’t quite know why, I think I was more in love with having a girlfriend than her herself which is a me problem) and I wish I was more blunt with some communication with her. I have some regrets but all my friends and family say that she was never going to change.

What are your opinions? Can people change?