A therapist perhaps would do the trick.
This is something that maybe you should have brought up before you got married. Why now are you losing sleep and can't eat? You kind of knew this going in. Whats the difference between 7 and 20. According to you it should have only been two, you and her ex? So what about the other five guys? The first five where okay, and the next 12 were not?
This is all you dude. The past is the past is the past. Why ruin your marriage and stress yourself out over a non-issue. And you wonder why she doesn't want to talk about it? Well, you try and pry out information from her that makes you think of her in a bad way. Things that happened before she even knew you. And normal human everyday things at that.
Lets say 1 in 12 people in the world today will have sex. Probably about of a quarter of a billion will be unmarried, or single. Think about that: 250 million single people are going to have sex within the next 24 hours. She didn't murder anyone, she just lived her life before she met you. So why would she try and make herself look bad in your eyes about something she no longer has any control of.
So I really think you should talk to someone who knows about human psychology way more than me. Otherwise you are going to throw your marriage in the toilet.
Good luck with everything. Get some help. This is really something small and not worth ruining things over.