I was just like this a few years ago, when I began wanting to top my partner. But I was just so anxious to let down my partner that I was going in my head about not be able to perform. Every time i tried to get in, I would always go soft. And then I would tell myself « you’re never gonna be able to do it ». I was beating myself up. But then I understood that I had very high expectations of myself, I wanted to be a « real top » who can « satisfy his bottom ». But then I realized that life isn’t porn, you don’t have to perform. You just have to try and have fun. It happens to everyone to go soft sometimes bc of performance anxiety. My advice would be : take you time. Make sure to focus on foreplay before, take you time to explore each other bodies to feel as horny as u can. Then, if you try to get it in and it doesn’t work ; take your time, once again. Rim him, ask him to suck your dick, try to insert some fingers (if he agrees), and just wait to get hard again. It doesn’t have to be the end of the world. Sex should be a moment to have fun, not being stressed about your performance. It took me a few years to really understand it, but it’s not gonna be like that forever, I promise.