I am sorry to learn that you are struggling with this.

When I read your question, my gut level response was "So what if she has had more partners?"

Now I understand that this is causing you pain.

Does your GF do anything to cause you pain in the here-and-now? Flirting, etc?

I think the idea of comparisons -- you are the best at this or that -- is bringing out your competitiveness.

Personally, I think people should keep their sexual histories private and go into relationships as new people. Saves a lot of wear and tear on the emotions. But that train done gone from the station.

Letting go of the past is often hard, and I am sorry that you are going through this. I hope you find the right combination to help you get past this.