sounds like a first time issue, it happened to me a lot early on. If you masterbate, you should probably stop so you'll be at your max when you're with your partner. Also when you do go limp, stopping and giving her oral might help as well. And condoms do feel different than the real thing, but its not worth the risk of a pregnancy (or even a pregnancy scare).

I generally do not reach orgasm during vaginal intercouse (when using a condom). The condom just seems to dull the sensation too much. Besides that, I'm usually focused solely on pleasing her, which serves as a distraction from the sensations I feel. Compare that to receiving oral or a handjob, where you're focused entirely on your own pleasure. However, I've never had an issue with going limp. That could just be a first-time thing though.