If there are certain specific things he says or does that he did with his ex, you could ask him to do something else. When my husband and I first started dating, I used to tease him I guess exactly how his ex used to tease him, so he asked me to stop because he didn't want to be reminded of her while he was with me. It wasn't a big deal, I just found another way to tease him without using the same kind of phrase she did. Is there something small he can change that will make you feel less like you've just replaced her? Just lovingly explain that you have been feeling jealous over his ex and you would like it if he did this certain thing a different way so you didn't feel like he treated you the same way. Explain that it's just something you're working through, and you need his help to not feel jealous about her anymore.
If it's just the whole idea of his past girl in general and not something specific he does/says now and he's not communicating with her, you need to deal with this in your heart, and decide if you can live with this long term. He did stuff for this girl and with her; can you accept that? It's not an easy thing to accept, but you can if you want to. Why not suggest writing a list of all the things you love about each other, what makes each other unique, and swapping? You don't have to mention jealousy; just as something fun to do for each other. When you see the things he writes about you, I think you will realize that he cares for you as a person, not as a replacement. Don't freak out if he writes less than you do; women are more detailed, while men think more big picture, so look deeper into what he writes than just the number of things he says. Men process things different than women.
Is he still seeing her or communicating with her? If so, that needs to stop. Don't be mean about it, but tell him that it makes you very jealous for him to be around his ex, so if he loves you and values your relationship please let her go and focus on the current relationship. Don't be accusing, just matter of fact and loving. Begin by telling him something he's doing right and end the same way so he realizes he's not a failure as a boyfriend, he just needs to address this issue.