I am certain that it is all in your head. Most problems people have with sex fall into that category. That’s not  to say that there can’t be physical problems, especially as people age, but it sounds like you’re young and so the cause is psychological. And you have even given a possible cause, which is that you’re under a lot of stress.  The solution is to clear your mind when you’re having sex, and one effective way of doing that is to fill your mind with erotic thoughts. Concentrate on some erotic fantasy. It can include your girlfriend, but if that’s going to be distracting, then it can be some starlet or a whole team of cheerleaders. The phrase, “Whatever turns you on” is appropriate.  Hopefully you’ll be able to keep your thoughts on this erotic material so that you’ll keep your erections. If you really can’t do this on your own, then go to see a sex therapist.