Mind is powerful : it can get it up just by daydreaming about a hot stud, or completely turn it off if it's stressed out ! I'd say erection and cuming is at least 75% mind.
It takes only one little bump on the road (lack of juice 'coz you jerked off too much for instance), to mess the whole thing up !
Well, 1st of all, go see a doctor, just to be sure there are no medical condition behind all this (chances are, there's none).
Then... well... get out of your head !!! The more you'll focus on it, the worse it'll get, typical vicious circle here ! Easier said than done, I know... Here's some tricks you could try :
- start any date / hook up by telling the guy you won't get far in any sex activity, even forbid him to touch you in any arousing way, just focus on having a good moment, talking, getting to know each other...
Chances are, it will help you lower your stress level, and you could end up having sex without any trouble after all ! But you have to trick yourself : trully believe NOTHING would happen, just have a good moment with the guy, and see where things go. If nothing, well... you said it from the start ! No big deal !
- identify exactly the context of your problem : can you get it up when you're alone ? What kind of stuff turns you on (so you can picture it in your mind when you're with someone, to warm things up)
These are typical tools / advices a sex-therapist would give, enjoy it, it's free ! :p