Sex shyness?

Hi! I'm 18 and came out about 6 months ago. Since I can remember I've always fantasized about guys. I've blown one guy before but have never had sex with men. I've fucked a couple girls in highschool, but it just didn't seem right. Now, when I actually have the chance to get down with a guy, I'm like suuuper hesitant. It's like it kinda scares me, even though I really want to. Has anyone experienced this? It's to the point where I kinda have to make up an excuse to why I don't feel like having sex at that moment, even though I've been trying to hook up with this person all night. The thought of me being asexual keeps popping up in my head, but I try to push it out. I'd honestly appreciate any feedback at all!