Rejection and abandonment seem to be a common theme.

I dont know if you care, but my close friend does EXACTLY the same thing. She messages Ex's that didnt end well, and then wonders why they "abuse" her. She always wants to contact people from years ago whos relationship with her didnt end well because "its sad to lose that connection" and she needs constant reminding that she is popular or has people who will reply to her desperate texts. She is working on it in therapy for BPD. Shes not yet ready to do DBT at this stage, she has a lot to learn about herself, although now she is aware of her behaviour, which it sounds like you are too.

Are you in therapy? I cannot relate to messaging ex partners but I do think it is normal to wonder about those in highschool. I loved my high school reunion for that reason.