Accidentally seen a girl my bf f****d

So long story short I was scrolling on a page that our college made and I seen this really pretty girl. I was shocked that she even went to college in our area because she looked like she belonged in Cali or something. I checked out her insta for some reason and noticed she had a lot of followers and was verified as well. Anyway I seen that my bf had liked her posts for a couple months straight(in the past) but he didn’t follow her. I wasn’t specifically looking for this, but his name popped up on the bottom of her posts so I just happened to notice it. I had remembered when we had first started dating, (this was kind of weird)his sister had randomly mentioned her bc she was trying to make me feel pretty or something and I said I didn’t want to hear about it or see what she looked like bc it’s not my business, but she had mentioned something really specific (physically) about this girl and something she was involved in, and this girl matched the description perfectly. She’s drop dead gorgeous and sometimes he gets soft when we do stuff as well. I just feel really bad about myself because her body and face is a 10/10 and I’m not sure what to do bc we have problems with intimacy a lot (which I’ve voiced bothers me). He’s also said that my body is just “normal” and he doesn’t ever pay attention to any of my body when we do stuff, he kind of closes his eyes the whole time. I’m not sure what to do but it’s hard to be with him when his past keeps popping up. This isn’t the first girl this has happened with, 2 others go to our college as well and I just found out on accident. P.S. I know he wanted to get more serious with her but she didn’t feel the same. Am I being immature?