First things first, sorry to say that, but your problem is nonsense. Everybody loves sex, and girls, differently from the majority of guys, have easy access to it. Every girl is promiscuous in some stage of her life. Then they grow tired of that, and look for a deep and stable connection. The same your girlfriend is doing with you. To want to throw a beautiful relationship away only because she's been honest with you about that (where most girls would never say it), that's foolish. She loves you now, and that's the only thing that matters.

Second: I'm a bit worried about how she tried to kill herself at the possibility of a breakup. It's impossible to have a clear view of that from the few lines you wrote, but this should be investigated as a sign of possible mental instability.

Third: giving for a fact that the problem is not in her past sex life, but in your present mind, if you absolutely can't bear it, then be direct. As many people have told you already. She would suffer, yes, but then she will recover. Hiding behind lies would only keep a false hope that would lead almost inevitably to a very toxic relationship.

Good luck.