Is this only when your with a woman? Or when you are by yourself too?
Seriously, I had an issue for about a week, where no matter what I did, when I was with my girlfriend, I'd lose it. Odds are, your putting too much pressure on yourself, and it compounds now that you think it's going to happen.
There are 2 ways to fix it that should work
1) Have no intention of having sex. Just both sit there (naked), and fool around. But, never have it get to that stage. After about 20 minutes (and no pressure that you're going to do anything), odds are you'll be fine. If not, no pressure, since you're not going to do anything.
2) Get a blind fold and blindfold yourself. Let your woman do whatever she wants to you then. Once again, 20 minutes of this, and you'll be fine.
It could always be something serious, but I doubt it. Just relax, and everything should be fine.