Not to pry, but I feel like this is a conversation that you and your boyfriend should be having with each other, you should feel comfortable around him enough to know that you can talk about things that are on your mind about your relationship, and what things you're insecure about. Keeping all of this to yourself without having a heart to heart with him will eat away at you and basically make you paranoid, but if you open up to him about this, there's a chance that he will open up to you too and you'll get closer to him knowing that you can emotionally be there for one another regardless of the past because what happened in the past happened in the past and will stay there, you and him are in the present together and you should be building your relationship with him, and living in the moment. You'll meet relationship milestones when they happen but right now you have to realise that everyone will enter a relationship with baggage and that nobodies past will be perfect.
A relationship is built upon trust, loyalty, and communication, which is why I am encouraging you to communicate with him and open up to him because it should bring you closer together, that's what it is about, building bonds and closeness with one another. He's with you and not his ex, and frankly, he trusts that you won't cheat on him or break his heart, so you should care less about his ex, and care more about how to make your own special thing between you happen, and work on that instead because the relationship he has with you is the most important thing and not what he used to have with somebody, let that go, and be in the past. Whatever special thing he had with his ex when they were together... that was destroyed by what she did, there's nothing special there anymore and you don't keep a part of someone in your heart after they cheated on you, then got with that person they cheated on you with.
Hope I've helped answer this question :)