Hi Joe,
First off you have determined that you are having a stress related problem. As you become more adept at stress management the problem should subside. One of the worst things you can do is worry about this too much. If you begin to doubt your erectile ability or feel that there is a certain degree of hardness to be achieved then you are likely to exacerbate the problem.
I like doc's answer a lot, especially the third paragraph. For practical purposes all I can add to that is to orient your foreplay activities around the things that turn you on the most. In my case the surefire causes of erection are breasts pressed into my face, a face full of muff and fellatio.
There are going to be days or short spans in your life when this sort of incident occurs. It is temporary and you will get over it. Just remember, don't dwell on it and don't feel guilty about not attaining the quality of erection you are accustomed to; that will only feed the performance anxiety. If you go for a couple of months without ever achieving a satisfactory erection then you might need to consult a specialist.
For now, relax, have your girlfriend give you some special attention and get your mind off work and sexual performance.