How do I stop obsessing over My ex boyfriend

I have borderline personality disorder which makes it harder.

We used to date, almost 5 years. We got together when we were in high school so he was my first everything and same for him. We were best friends, practically one person. We were going to get engaged a year later.

Almost two months ago he broke up with me. It was unexpected. Ive begged him so many times to come back and he has rejected me repeatedly.

I want to move on with my life but simply cant. Cant stop thinking, crying, running, reminiscing, etc.

Please help….