Are you fully comfortable with the idea of sex with a guy? Sometimes latent issues that we heard growing up can play with your head (both of them!). I remember that when I was young and on a swim team, we had to go and get Speedo swimsuits. I had never worn one before. My mom made a big deal that they were too revealing on me. That programmed me to believe that I had to hide a part of myself, because other people should not see it.
I was a virgin until I was 21. I went through my entire college career celibate. I could not get over the idea that no one really wanted to see my cock, and certainly not in a sexual situation. It took me quite a while, and many partners later, to finally get my head straight, and realize that it was my mom's hangup, not mine. Even when I was invited to a guy's place, I had erection problems because something in the back of my head kept saying that maybe he really didn't want it. It didn't matter that he was ass-up on the bed, begging me to stick it in. That's what psychological issues can do.
It's good advice to find a therapist, or counselor, and talk through your issues. And by the way, being a passive partner (bottom) if you feel you really want to be active (top) is NOT the answer. That would be settling for something you are not interested in. Why would you do that to yourself?
You can get through this, and probably fairly easily, talking to the right person. Then, you'll have a rock-hard cock the next time you're with somebody, and really enjoy your first time. And your partner will too!
Good luck.
P.S. I still wear Speedos, and I am proud of how I fill them out!
