Frustrating ED

So, I do not know what my problem is. I thought I figured it out and solved it, but after last night clearly I didn't. When I'm with a girl, I become erect very easily, and usually stay erect for awhile. However, once the girl takes out my penis I don't stay erect long at all (maybe a minute tops) and I do not ejaculate. A lot of times my penis goes soft and then a couple minutes later it will become hard again, then the girl starts playing with my penis and then it will go soft again. Last night the girl said, "I don't get it, you keep getting hard when I'm not touching you, but then when I touch you, you get soft."

Clearly I am turned on by the girls otherwise I wouldn't become erect. So, it is really frustrating not knowing how to solve this problem. I'm 20 years old and in good health, so this shouldn't be happening to me.

Does anyone have any advise for me?...Right now I think I am going to try and give up masturbating for at least 2 weeks. Other than that I have no other ideas.