This is unfortunately a situation where you need to either accept it or not.

What you need to understand is that this is an absolute non-issue and you're potentially throwing away a great relationship (if that's true) over someone that has no impact on your relationship.

I'm not sure if this will help or not, but anecdotally, I'm a guy and I'm quite happily married. If I'm being honest, I wouldn't objectively say that she's the best sex I've ever had; however, sex with her is absolutely amazing. If she were asked, I doubt I'd be the best either, but again, the sex is great. But am I sitting here ever thinking about them or comparing them? Nope. They're the past. Those guys your girlfriend was with are the past.

Again, you have to believe that she enjoys sex with you, or she wouldn't be with you. You need to be confident in that.