How do i get s when it counts?

First off im 18, and i can get eretions fairly easily you know, while watching or seeing attractive women on tv and getting nasty thoughts even. But when my girlfriend and i engage or are about to in ual activity it is much harder to get them for some reason, im thinking its nerves or performance anxiety but not exactly sure, it could maybe be the fact that i don't enjoy wearing condoms (she is on the pill but we choose to use condoms also). Were both completely comfortable with one another which is why this is so frustrating for me, and her. We've been in a relationship for about 6 months so is nothing new for either of us, and i mentioned her giving me head but she doesn't enjoy doing that (she has tried hand jobs to get it up but even that doesn't seem to do the trick).. It gets fairly erect but not enough to put the condom on. I guess what im asking is what are some ideas to help us, or if there are any vitamins i could be taking if it were a malnutrition problem even? thanks in advance