Suddenly unable to get erection.

I am a 25 year old male. I recently entered into a relationship and the first time we tried to have sex I couldn't maintain my erection. Since then (about a week ago) I haven't even been able to GET an erection. If I do it is still pretty soft and only happens as a result of self-stimulation. I have never had this problem before and I can't believe it is just setting in now. Could this be psychological? I am very upset and stressed out about this. I am a little bit overweight but am currently dieting and exercising regularly, and never had a problem getting an erection before so I don't think that is the problem. This literally started THE FIRST TIME we fooled around. I am extremely attracted to this girl and absolutely love being with her. Sexual attraction is not the problem. Also before entering this relationship, I had been single for several years and had masturbated regularly. Should I stop doing that? Might that help? Any ideas?