Re: wont stay hard with condom. but fine without
It sounds reasonable that this happens. I can remember it was much less arousing to be "suited up" -- I couldn't feel as much, and it didn't feel as intimate.
Things to try: lubricant inside the condom, around the head of your penis. Or, try lots of foreplay, have her masturbate you (anything but penetration) until you're desperate to ejaculate, and THEN put on the condom. Or, try a different style or brand of condom, it might be too tight.