I find that not knowing things leads to stress: you answer your own questions, you fill in your own blanks, about what your partner and his exes shared together. But you can never know if you're filling the blanks in correctly, so your stress never goes away. There is a huge ? hanging over his ex's head when you see her.
DP and I lived together for years as friends before we became a couple, so I know every girlfriend he had in those years. He knows the boyfriends I had. We overheard each others' sex lives and rows. (One of his exes made really stupid squeaky noises.) We cried on each other whenever we got dumped. We know everything.
We were jealous of each others' relationships then, but I feel no jealousy now. There are no unanswered questions, no blanks to fill in.
Filling in your own blanks, especially when you have an active imagination, is the killer.
So maybe we have to learn not to hurt ourselves, trying to answer our own questions. If you don't know something that you can't ever know - what good times did they share together? what promises did they make to one another? did they have sex in positions we don't? - then all we can do is wonder, and worry, and fill in blanks with answers that can't ever be accurate. Just recognise that you're hurting yourself by trying to answer the unanswerable, and that it stops you living in and enjoying the present. It is a use of your mental energy and your imagination that is working against you and making you sad.
Whatever they said, or did, together, it wasn't right for him. His past relationships are past because they were wrong. He hadn't found you yet, but he was on his way to you.
He's with you now. He's with you because you're you. He wants to be with you. What you two have is yours. Don't let his exes be in it too by allowing them space in your mind.
When you see an ex of his in the street, just imagine the word NO above her head and the word YES above yours. Your YES has flashing lights, and her NO has a sad trombone noise. He loves you, because you're you, you are the YES, and you're great. 