Hi there.
I have felt the way you are feeling many times before with my current boyfriend. I find myself looking at pictures of him and his ex, any over analyzing everything- like how happy he looks with her etc etc.
I have also compared myself to other people he has been with, thinking that maybe I need to be more like them...
When I was like this, I was having problems with anxiety. I do not have mental issues, it was just a matter of being insecure, which I still am at times, although my jelousy issues have gotten better, for the most part
Everyone has a past, but it is just that-the past. Don't torture yourself by constantly looking at pictures of your bf and his ex, and don't analyze them or think about his past if you can help it.
You have to remember that he is with YOU now, and his relationship with her is in his past. I wish you all the best, and take care

Feel free to private message me anytime you need to talk.
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Positive affirmation: I have a lot to be proud of and a lot of good things in my life