Am I overreacting still? (Please answer)

So my bf and I have been together for 1 year and 2 months. I know he loves me and he proves it all the time and this is literary the best relationship I’ve ever been in. However I still can’t get over the fact that he was infatuated with this girl for a while before he met me and then he was kinda talking to us at the same time. This girl is a year ahead of us and already graduated and he took her out of his life to prove to me he doesn’t care for her yet still looks her up on Instagram? I think I’m more bad about the fact that he was talking to both she and I at the same time and if she didn’t stand him up then we wouldn’t be together. He told me that he did have feelings for her for a while but stopped when he started getting involved with me but idk I still feel like he’s lying. Someone please help.