My boyfriend (24M) frequently brings up his previous relationship.

Hey, I've been dating my boyfriend (24M) for about a month now, and I’ve noticed that during our conversations, he often references memories with his ex. I understand he was with her for two years during college, so it's natural he would have some lingering thoughts. I've also moved on from my ex (23M) after a four-year relationship that was quite toxic, but I haven’t mentioned him once. For instance, while we were at a party, my boyfriend said, “When my ex and I were in college, we knew this one guy—” which seemed unnecessary to bring up. Later that night, after sharing a few drinks, I asked him about their breakup, and he casually mentioned, “She moved for her job,” before dozing off. Even though it's been two years since they split, I can't shake the feeling that he might still have feelings for her, and I worry that I’m just a stand-in. In short: My boyfriend frequently brings up his ex, and it makes me feel like I’m just a replacement.