I’m not a guy, so I don’t know much about the “first love” thing. But I know with us girls, it’s hard to forget your first love. Hell, I still haven’t forgotten my first love. He’s everything and more still to me. Even though we’re not even friends. And he hates my guts.
All I have to say is, just let it go. Don’t make it a big deal. Cause seriously, he’s with you now. Not her. She was the one who won his heart first, and it’s going to be hard for him to forget her. But, give him a reason to want to forget about her, and just focus on you.