It is definitely a mental difference whether one realises it or not. Actually have had the same booner problem for like 5-6 years. Whenever I was with a girl, specially a new one it wouldn’t get rock hard, hard enough for penetration and fun, but not the rock hard one I want to show off :P actually the first time I had sex it took me like 30min to cum. First I thought anxiety. But after a while I realised that anxiety didn’t really apply to my nature. What iv figured through heavy meditation and soulseeking is really silly simple.
It’s like when your going to bed and your saying to yourself now I’m going to sleep, fuck you never going to fall asleep :P but if you’re tired, going to bed, relax and schmack you sleep. Like when you’re jerking of you’re concentrating on the pleasure in the penis and not going over the edge, try doing the same with the girl, feeling her moves, moans body and every inch of her pussy. Might be easier being this egocentric if you make her orgasm a couple of time first with your tongue and fingers, so you won’t have to think so hard on pleasing her and the performance of penetration. Plus it makes her pussy more responsive to slow and shallow strokes, which may help increase the time of penetration.
My ideology is mental one, physical another. In other words a large cock is only half, the rest is tantra :D