I'm 17 And Have Been With My Girlfriend For 4

I'm 17 and have been with my girlfriend for 4 years now and we are both virgins. I do masterbate a couple times a week and can get almost a full erection when doing it. When we are just kissing and hanging out for awhile I can be almost fully hard for hours but as soon as we go to touch or I take my clothes off I go completely soft. Please help