People asking clarifying questions:
We're in our 30s and have been dating for about a year and a half. We've always had problems with trust in our relationship. Not for cheating or anything like that, but because he wasn't honest about his past with me, and because he was pretty terrible at honoring commitments when we started dating. Inconsistent behavior and half-truths led to my distrust, but we are working on this.
Of course, something like this wouldn't have bothered me if we had relationship with minimal arguments, but we have been rocky for the past few months due to the issues above. I'll also be candid and say I don't appreciate the comments to the effect of "stop going through other people's things" -- hello, we live together. It was in our shared desk.
Like I said in the original post, I don't plan on bringing this up to him because I think retroactive jealousy is something I need to deal with on my own, but I understand why some people are suggesting that I do. But thank you for the people who validated that it's normal to feel uncomfortable (and like I said, I know it's normal for people to keep memorabilia as well). Looking at the brighter side of reality is super helpful, as some of you pointed out. He's with me now, and we are living together. He's happy with me, and we have very deep feelings for each other.
If any of you also struggle with jealousy and have methods of coping with it on your own, please let me know! I am not going to let this add to the pile of things we argue about.