It’s not your dick dude, it’s the emotional thing you both are going through. You can’t have healthy sex when you are emotionally unhealthy. She seems to have some issues and in turn, it’s rubbing off on you too. Or it could be you have an issue with her not having an orgasm so now she feels pressured. Either way, you both gotta fix what’s inside your heart first. Other things fall into place after that. Don’t trip about the guy with the big dick in the next room. Performance is everything. He can probably perform well because he is confident about his big one. You have to get your confidence up too. If you plan on being with this girl for a long time, get to know her body. Do more than just go down on her, stimulate her mind,.make sure your breath and body smells good. Learn her body as well as you know hers. You get that knowledge, and even a finger sized peepee will get her off, and give you confidence. Jmho.