My boyfriend, who is 21, is struggling with my sexual history as a 23-year-old woman. Will things ever improve?

I’ve been in a relationship with my boyfriend for nearly a year, and he truly is wonderful and treats me incredibly well. However, he struggles to come to terms with my sexual history from before we started dating, which leaves me feeling lost. He is my first boyfriend, and prior to him, I had three sexual partners, each encounter being casual and occurring during my first year of college. At that time, I was trying to navigate the hookup culture but soon realized it wasn't for me, and I didn’t have any sexual experiences again until I met him. Notably, he was a virgin when we got together. He really dislikes the fact that I had casual relationships. Since that lifestyle isn’t something he relates to, he finds it hard to accept that I was intimate with other men without any emotional commitment. It doesn’t seem to matter to him that it was years ago or that I was simply figuring things out; he gets overwhelmed with negative feelings about it every few weeks. His mood often shifts for a day or two as he fixates on my past. He insists he’s not judging me but admits it bothers him deeply, making it feel judgmental in the end. He also becomes insecure when I tell him that he’s the best partner I’ve had. I’m at a loss for what to do, as this situation is taking a toll on me. I feel judged and uncomfortable whenever my past comes up. I know he wishes he didn't feel this way and hoped his feelings would lessen over time, but they seem to be intensifying instead. I’ve tried to support him in various ways, but nothing seems to help. I’m starting to wonder if our relationship is ultimately at risk or if his feelings will eventually fade.