Tips to get over your boyfriends past

Title says it all.

Bit of a back story: my boyfriend and I have been together for 3 years, however, for a year prior to dating we had a thing that was on and off during that time we both slept with other people (so I’m totally not innocent) anyways.. the idea of him being with these other people is really eating away at me and for some reason is only starting to bother me 3 years in 🙃

The only reason I could see why it’s bothering me now is because I’ve only learned that he slept with these people 2 years into our relationship... I trust him and have no doubts he’s cheating however i seem to obsess over his past and the idea of him with these girls..

I personally think it’s a self confidence thing with me as my self esteem has really hit rock bottom over this year (not sure why)

So I’m just looking for tips on how to not be so focused on that past as yes I know there is no reason for me to be mad because we were not together during the time...

Has anyone else had this?

How do I make the thoughts of his past stop?

(I’ve talked to him lots about how I feel and our communication is good but the thoughts don’t seem to stop)