Talk to Urologist on Sexual Anxiety

I'm a sexually active gay man. (Brief background just in case - The active or penetrative partner is called the Top while the passive of receptive partner is the bottom). I have only ever been a bottom until now. However, this has affected my self-esteem and I now wish to be able to be a Top. The only problem is - Every time I try to penetrate a man, I lose my erection. I don't have ED as I get healthy and strong erections otherwise (morning wood, during receiving fellatio and at various other times). But just as I try to penetrate, the erection goes away and refuses to come back. I'm certain its a psychological problem and would like to see a doctor who would be willing to help out!