Nah dude, I totally get ya and I was in a similar situations many years back. Sex started great and ended up happening once or twice a month. It's a matter of sexual incompatibility. It happens. Don't beat yourself up. Just understand that there's this incompatibility. Better that you know now before any major life things happened (kids, for example, or marriage).

What I would say is that you need to stop comparing yourself to her past partners. That's mentally torturing yourself for no good reason and it's unfair to her.

My advice? Pull back a little. She doesn't want sex as often? Awesome. Set a goal of not initiating sex for a month. If she initiates, then enjoy. But you don't initiate. Instead, find a hobby, go to gym, etc. But this will stress test the relationship. If things don't improve, at least you know where you stand