Very demoralizing sex life, please help...

I can't seem to get my girlfriend of two years to have an orgasm during sex. This is extremely demoralizing and making think we aren't meant to be together though otherwise things are great. I can get her off very easily with foreplay or after I've finished by using my hands, but almost never during sex. I have a small penis (5" when fully erect) and she has a fairly large vagina. I've been trying all sorts of different positions but nothing is working. I should mention that I know most women can't get off during vaginal intercourse but my girlfriend has said she never had a problem with it before me (which really suck). Is this likely a problem that will never go away and we need to face it now or is there something I, or we, can do?