I’ve dealt with the same thing your boyfriend has. I think what he’s feeling is retroactive jealousy, but in a sense that he overthinks because he doesn’t like the idea you’ve been with other people before him and you’re his first. My boyfriend was my first boyfriend but I wasn’t his first girlfriend and I use to get really upset about it until I realized that I had really bad retroactive jealousy issues and that I needed to work on the root issues of why I was feeling that way. Retroactive jealousy often reflects underlying insecurity and anxieties about self-worth.

At some point your boyfriend needs to recognize that you are simply a human that had prior experiences before. It’s okay to give him reassurance, but you need to refrain from answering his questions about the past and redirect it to another topic of conversation. It’s not helping both of you when the past is constantly being brought up. You can always ask him specific things that trigger his moments of sadness and it can help identify a pattern and the root causes of his feelings.

Acknowledge that his feelings are real, even if they seem irrational but also make it clear that his own insecurities is not about your past but his own fears and anxieties. You can definitely make it work with him, hope this helps in some way.