Make it not important.
Like many others, I've also suffered from this, but fortunately I've had a very understanding and caring girlfriend to help me get past it. It frustrated me immensely, because I really really wanted it to happen. I broke in cold sweats while I was desperately jacking myself off, just so I would get hard. It didn't happen.
Fortunately my girlfriend made sure to tell me that it wasn't important at all and that all she cared about was being with me. When we were laying next to each other, with my non-erect penis, after a while I'd 'check' to see if I could get hard yet, but every time she simply slapped my hand away and told me to stop worrying about it. It wasn't important. What she did was she'd just put her hand on my dick and softly caress it while we made out or talked. It wasn't sexual. She just wanted to touch me and she said she liked the way it felt, whether it was hard or not. ...It wasn't long before I'd get rock hard in her hands.
From that point, we could work and improve. I mean, it didn't solve it. The first time she got me hard like that, I was all, 'let's fuck right now!' and as I got up and got on top of her, I went soft again with seconds. The second time we finally had sex. The third time she was a bit dry and when I didn't slide into her after 5 seconds or so, I worried and went soft again. The fourth time we finally tried a different position and as we moved around I got anxious again and went soft. The fifth time I couldn't get hard at all again. And so it went for over a month or so.
She was a trooper, though. Never did she complain, never did she make me feel bad about it. She kissed me when I couldn't get it up and would tell me she loved me very much. She's the best.
Since then I've gotten past it. A while ago we had sex-weekend and I came ten times. She's insatiable and so am I. I still go soft during blowjobs, though. I still struggle with the mentality that I have to be as hard as I possibly can, or else I might disappoint her or make her feel bad about myself. I think I'll always be like that, but with a girl like mine, I can do anything.