So me and my girlfriend have been seeing each other for 9 months now. I'm 18, and before her I was very sexually active, having many partners in the past and always was very confident with my sex. All of a sudden, 5 months into my relationship, my penis decides to randomly get limp during sex. It doesn't happen all the time, but it happens on occasion and bothers me because I'm used to being confident with my sexual encounters. I ruled out an std as I have been to the doctors before and they said I'm clean as can be. I have been getting 5 hours of sleep for the last few months, and I having been fighting with my girlfriend a lot, maybe stress is an issue? I'm not generally one to pass down sex, but for some reason my penis becomes limp during sex, and it bothers me a lot when I think about it. What should I do to alleviate the problem?

"Clean as you can be" sorry you are either completely free or you have a STD, there is no between. Clean as you can be with multiple partners over a few years ( my guess several with no protection) can lead to very unclean. But most STD don't effect performance.

Most likely lack of sleep and fighting with your girlfriend, esp if he is her you are having sex with.
Then what happens, once you have a problem once, you start to worry about the problem, and that worry makes the problem worst.