If you were just having problems with hand and blow jobs I was going to say that through your masturbation you've "trained" yourself to only respond to one kind of stimulation. But since you're going soft during/before sex I'm going to have to agree with Lilith and say the problem is in your head. The arguement could be made that you aren't ready for sex and the inability to orgasm or keep an erection is your subconsious' not so subtle way of telling you this.