Losing Erections

Hello, I am 20 years old and have masturbated since i was about 14-15 years of age. Almost everyday, a few breaks of a couple days or so, but you get the point. I first had sex about a year ago and it lasted for about 10 min, and every time i had sex i couldn't make my erection go away. I had an erection almost every time i saw this girl. Now when I have sex with my current girlfriend, my erections seem to go away instantly. She is very very good at stimulating me until im rock hard, but I lose erection as soon as I go for penetration! Its very frustrating and bless her heart she keeps me sane, and I want nothing but to please her like no man could. But I'm wondering if my over masturbating is causing this, and if my sudden rise in blood pressure is contributing, or is it a mental thing?? Please help me!!