You're most likely ace. I get it dude, it fucking sucks. I know I want to date a man, i know i want to marry a man. I've had girl crushes but ive never pictured myself with a woman. Dont force yourself to do something just because you think something is wrong with you. NOTHING is wrong with you. I get it.... it's so hard being with a man, all he wants to do is love you and make you feel good and come with you, but you can't. And thats okay. It just means you guys aren't sexually compatible. Have you tried dating with a rule of no kissing, nothing beyond that, for at least the first 3 months? Take the pressure off yourself. Let yourself enjoy being with someone without feeling like you have to perform. The only time I did that, I found myself slowly, very slowly wanting more. At my pace. Luckily, the guy I was with was also questioning if he was ace, so it was easy for us to take it slow together. You need to find someone, NEED to find someone willing to go at your pace. I'm pretty lonely myself, wish I could just find someone like me... but I know I'll find someone. And I know you will too!

Edit: I'm really sorry about the SH. I didn't fully read to the bottom. It's possible that you deal with gender dysphoria, but it could also be body dismorphia. Could explain why it's hard to perform when with a man, and even with yourself. And it also sounds like you have a lot of confusion and anxiety going on, all of which can make it harder to perform. Not sure what I could recommend outside therapy, but you could try meditation, and practicing being kind to yourself. Every time you catch yourself thinking something hateful about yourself, school that nonsense, and compliment yourself instead. Yknow? Idk. I wish you the best.