Weak Erections and Premature Ejaculations

Hello. I am 16 years old, and have been in a relationship for 4 months. Me and my current girlfriend have decided to enjoy premarital sex together; however, every time we are together and we start getting "hot and heavy", I can't get it hard enough to penetrate her. I am not completely in-expierenced, as 3 years ago I was in a 10 month sexual relationship with another girl in which we engaged in intercourse over 50 times within that 3 months. Since then, I have also had sex once with another female almost 1 year ago. I went through a month to 2-month period in which I had no sex drive at all previous to the relationship I have now, and on the first date with the girlfriend I have now I had a severe case of "blue-balls". Since then, I've masturbated, sometimes twice a week, sometimes almost three times a day. Since me and my girlfriend have started engaging in sexual activity, I have only gotten a hard erection once, and we were not in a proper place to have intercourse (also, we were both almost fully clothed, but I pulled her shirt up and her panties off under her skirt, and my pants were pulled down and we "grinded", but did not have sex because we were at school).

I am scared I have a physical problem, because even when I masturbate I don't get a rock hard erection and I cum within a couple minutes, and the same happens when I am with her. I am too soft to penetrate, yet I am able to cum.

I have not talked to my parents yet, as they do not support my engaging in sexual activity, but if I need to see the doctor I can probably get there some how. I've mentioned it discreetly to my girlfriend, and she doesn't seem to have a problem with me, but neither of us are really sexually satisfied like we should be.

Any help would be appreciated!