How do I stop thinking about this?

I'm in a new potentially serious relationship for 5 months now. We get along really well, have great communication and got so much closure. We're 21&24 and we both study in med school. The only problem is that I get so jealous when I think of his past. His flings, flirts, or exes from before we got together. I've got a pretty complex past too but when it comes to his I just feel so jealous. (I don't mention it to him though, I just crumble it within myself). I know we're together now and he's totally made it clear he's never been like this with anyone before, even his friends have mentioned to never have seen him this happy, BUT I just get really upset when I check his social medias (twitter) from months & months before we've started talking and find any kind of flirtatious comment or anything of that sort. I know it's the past and it shouldn't bother me this much but it really does...😖