Trouble getting and keeping an erection

ok, so ill admit to my pre-pervert status. Ever since probably 8th grade, ive been a frequent masterbater. I remember at first when i started to experiment, i could get turned on and get an erection from the simplest things. Ill admit, i probably jerked off at least once every two days, usuing mostly sites like red tube and what not, and now i find it extremely hard to get turned on by anything, videos or pictures, and it was all normal until this past january. I have acquired myself a girlfriend now, and when we would hook up, id be rock hard, but thats with my pants on. When my pants came off, my erection literally dissappeared. And ill admit to this one as well, i have the kind of dick that goes from like 1 and a half inches soft to about 6 inches hard, so there has to be a lot of blood being pumped to my dick for a full on erection. Even though i can get a ***** while we are hooking up, while shes not looking at it, its not at its full potential. I havent been able to get my full 6 inch erected dick for a while, i cant even remember the last time i did while watching porn. Im scared cause i dotn wanna have to use something like viagra for the rest of my life. Im 16 years old, i dont know what is wrong with me. I was thinking maybe i have become immune to porn and what not cause im so used to seeing it. I have stopped watching porn entirely, for about two months, and it hasnt really helped. When i was younger i could of thought about any random girl and achieve an erection. Now i cant send any blood to my dick by thinking about anything. Im really scared cause its embarrassing. I have talked to my girlfriend about this, and she always blames herself, and i feel terrible because all i wanna do is make love to her, but this problem is making it tough. Last night she was giving me a handjob from under the blankets, and id say i was pretty hard. But when she went to give me a ********, i lost my erection, I dont know why. I guess nerves have to effect this, like im nervous or something without realizing it, but that also doesnt explain how getting an erection when i wanna masterbate is really tough now, and i can never truely feel turned on any more, its really sad and pathetic, SOME BODY please help me, I know this has been extremely long and tedious to read, but i really need your help. Should i visit a doctor, i mean im only 16, this shouldnt be happening to me.