Small Penis Anxiety

My girlfriend and I had a discussion about how many people she has had sex with. Her number is way higher than mine and I know just by height of some of them that their penises are probably way bigger. Every time things are starting to get hot and heavy I freeze because all I can think about is her with guys that pleased her better. It's ruining our sex life and I can't bring myself to tell her why I get so weirded out sometimes. Sex was fine before the conversation, what should I do?

You are wayyyyy too paranoid.
Did you just say that you can tell other guys' penises are bigger by their height? Are you kidding me? My penis is bigger than most of my friends', and they're taller than me. How do you explain that?
I don't know if you play sports, but sex is like baseball. Don't think, just play. If you're out in the field thinking, "I hope the ball doesn't get hit to me," it's gonna get hit to you. If you're gonna have sex thinking, "I hope I do well, I hope I'm doing everything right, I hope I please her," then you won't.
Stop thinking about it. Shut your brain down. She's your girlfriend, isn't she? Your penis isn't going to get bigger, is it? And even if these dudes had big penises, it doesn't mean they pleased her better. Size does matter, but it's not the only factor when it comes to sex. Sex is all about making the other person comfortable, making them believe that nothing else in the world exists except for you and your penis. But for you to do that, you need confidence. Unless you're hung like a thimble, there's no reason for you not to be confident in your skills. This girl is dating you because she likes you; don't fuck it up because you're too self-conscious. Sorry to be so frank, but I think you need it.
Next time you're about to get physical, eat her out for awhile, tell her how hot she is, then show her what you're capable of.