20 years old with psychological ED?

I am 20 years old. I am in college, I exercise regularly and eat well and am in very good shape. Lately I have been having problems with psychological ED with my girlfriend (my first girlfriend, be it at that). Every time I am about to lose my virginity, my erection slowly fades away. At first I thought it would be nerves, but I do not feel nervous with her. Or perhaps it’s from all the porn I used to watch, but I stopped watching porn and masturbating a few months ago. I need help. What could it be? I am able to get an erection by myself, but in bed with my girl I cannot maintain it long enough to reach penetration. She says she’s fine with me “taking things slow” but I can see the disappointment in her eyes and I’m afraid she will leave me eventually. We have been dating for a month or 2 now. What is wrong with me?