OK, this is gonna sound dumb....but you said you were just starting...so maybe it's worth a shot.

If the condom is too tight, it'll cut off the blood supply, and then there's no way in hell you can keep an erection. Don't know what size you are, what size you're using, or if that's the problem. More likely it's one of the things that Lou mentioned. But thought it was worth throwing out there just in case.