I can't keep an erection?!

I can't keep an erection?

I am 25 and I have been with two women. I have recently started dating a new gal and she's great. Problem is, I can't keep it hard. I am anxious about disease so we both got tested. Now I am still anxious about disease, and about getting her pregnant even though she is on the pill. I can't get it hard even with a condom. If it does get hard and it stays that way long enough to get it in, I orgasm in about 2 seconds. She is very patient and understanding, but how do I overcome this? I am pretty sure it's all in my head, because I can get it hard other ways but when I am with her it just doesn't work. help me!

masterbate before hand and lots of foreplay.