Can't achieve/maintain erection with girlfriend.

First off, I am 22 years old. I've been seeing this girl for just going on 3 months now, and since the beginning, I've intermittently had issues maintaining or even achieving an erection. At first, we both chalked it up to anxiety. It would occasionally happen, but the ration of success to failure was generally tilted in favour of success. When I have been able to achieve, the sex is generally very good. 

However, the last three occasions we've attempted to have sex, I have had issues maintaining. The first time, I couldn't get hard; she had to get me there herself. The second and third times are what concern me the most. During foreplay, everything was fine. I achieved an erection, and i was ready to go. The moment we disrobed, and I gathered the condom, the erection began dwindling. What is wrong with me? I'm crazy about this girl, and I can feel this problem starting to put an unwanted strain on things between us. She's questioning whether or not I'm attracted to her. I think she's beautiful. Is this a medical issue? I'm only 22. This shouldn't be happening.