Using a throw away acct because my partner follows me ...anyway

My boyfriend doesnt usually use social media. He hasnt thru out our entire relationship. He recently reactivated his fb and added me. I noticed a ton of pictures with this same girl. Like a lot. I figured based off the amount of pictures and the body language (her hand on his knee, arms around each other etc) this was obviously an ex. I asked him about it and he told me they are just friends...I think this is strange because he has literally never brought her up. He confesses to me he use to have feelings for her but she turned him down and nothing ever happened between them. This is a couple months before we got together so it shouldn't bother me but it does soooo bad. My boyfriend is extremely good to me and never given me reason to doubt his love for me but I can't help but feel slighted. Sometimes it literally feels like he cheated on me in these pics even though it was before our relationship. I know this is irrational and very petty. I thought I was moving past it but I just noticed she heart reacted a recent pic he uploaded and it sent me back down the rabbit hole. Now I feel convinced they have been reconnecting on facebook. Since he just reactivated his fb there are only two pics of us together which I feel like he threw up there out of pity and a plethora of the two of them that keeps me from even visiting his page. Any advice on how to self soothe myself about the situation so I don't fuck everything up with jealousy?