It is often the case that someone we fall in love with has had prior relationships, even marriages, and while the pain of knowing they have been intimate with others may be uncomfortable, it is part of being human that we are built to adapt. When one partnership doesn’t work out, another one can flourish.
I recommend that you try to view her past as leading to your current and future relationship. Had either of those other two (or 20 of yours) been satisfactory the two of you would not be together now. Our past moves us to our present and the dissolution of those relationships allowed for the two of you to be together. You could work on honoring this truth rather than staying stuck in an unchangeable past. If you do talk about the past talk about it as a learning experience that both of you can grow from, rather than something that can harm you.
Whatever went wrong in those relationships gives you a road map of what not to repeat in this one. My guess is that had the two of you tried to stay together when you were 16 it may have fizzled. Try to understand that experimenting with other relationships was necessary for you to turn your attention back to each other.
I would give this 30 days of trying to change. If by then it hasn’t you may want to check the “find help” tab at the top of this page for therapists in your area.