Trouble having sex.

I'm 18 years old and can't keep an erection. I started masturbating when I was about 10-11 and I have never really used the conventional way to do it. I also masturbate dry (no lotion) if that matters. Now I masturbate at least once a day usually but I'm trying to cut back. When I do masturbate the only thing that can get me excited enough is porn. Not even looking at pictures of girls I think are really sexy. And even with the help of porn, once I'm just starting, I get hard but over a couple minutes I go from about 95% to about 70%. Flexing my legs while I do it helps, but I try not to do that as much as I can now because my body has gotten accustomed to that and it seems to be the only way I can cum. So with the limitation of flexing my legs and the lack of being able to keep my penis erect for a long amount of time, it has really turned me off from trying to have sexual experiences with girls to avoid embarrassment and disappointment. The only time I'm not afraid to attempt something is when I'm drunk so if anything goes wrong I can just blame it on the liquor, but obviously the liquor doesn't help me with problem and in most cases makes it undeniably worse. I'm not a virgin, and the 4 times I've had sex I wasn't able to cum during and 3 of those times I was under the influence of alcohol at different levels of inebriation. I have also received oral sex for a considerable amount of time and wasn't able to bring myself to an orgasm. I'd really like to nip this in the bud as quickly as possible and fully enjoy the rest of my college years and finally look forward to having sex for once in my life. And I know this is pretty pathetic...but it's my life, and I need some help. And please don't tell me it's just some kind of psychological problem, like inexperience, without giving me someway to overcome it. ps. I also think that it has gotten worse over the years. I don't even wake up with a hard on like I used to everyday. Just trying to give as much info as I can. Thanks for taking the time!