Yes this sounds like performance anxiety and its important not to be too worried although I know its upsetting. Try to focus on other things at the point you're ready for intercourse. Have you talked about this with your partner - her acceptance, reassurance and patience will help you overcome this. Its sounds like the condom started this and its snow balled as the other poster said.

What do you do when the erection stopped - did you stop foreplay and sex? If it happens again, don't worry about it and continue pleasing your partner. I know you said you don't drink but don't think this will relax you. I wouldn't go the little blue pill route just yet and if this continues tell your doctor - there are simple beta Blockers that help with performance anxiety and anti-anxiety meds. Take the idea of "performance"out of the picture and look at the whole experience - enjoy foreplay so your partner is still happy and enjoy feeling close. its okay and will get better if you don't stress about it. Spend a few night just touching and feeling close but no intercourse - this will allow you to relax but still bond. Good luck.

How To Conquer Sexual Performance Anxiety | LIVESTRONG.COM