Don't feel anything in my penis, can't keep an erection.

Hello

I'm 18 years old, and have some sexual issues. I've been masturbating since I was about 11, and what I did was lie on my belly and push my penis to stimulate it. When I did this, it felt good but it was always soft. I've continued this habit because it felt good and never thought it would be bad. I've always tried to jack off but it never felt good so I always went back to my method. I now have a girlfriend, and the other night we tried to have sex but I couldn't keep an erection up so we didn't. I felt bad and ashamed because I couldn't keep an erection. I masturbate frequently, and I haven't really been turned on in a while. Basically to sum it all up, all my life I've masturbated with a soft penis, could never get into jacking off and now I can't get an erection during sex. I heard you can "re-train" yourself and I think that's what I'm going to do. I'm going to stop masturbating with my method and stop masturbating in general. I'd imagine after a few days I'd begin to get bad urges, but I have good self control. Anyways, the only way I'd tried to masturbate is with a soft grip going up and down, or when my girlfriend does anything to do. That way, I stop my method and I hope after a while of not getting pleased my penis will get use to the new normal method and begin to pick up on it.

Any advice?

Thank you.