Well, I'm a student, so my situation isn't quite the same as having a 40-hours stressful time. Right now I work 30-35 hours at a back-breaking (because I have to be standing all the time) job, though, and when I come home I'm exhausted. And yet on most days I take a hour to eat and relax, then I sit and write until I've hit at least 1k. It's either that or I got up early to sit and write (my work schedule isn't stable).
I don't think there's a miracle solution for this. It's often hard to start up the machine after work, but about 30 minutes into the writing, I find my pace again. It's easier when I do it everyday, too, because the last scene is fresh in my mind. One of the reasons this works for me is that I've established a few "routine markers". When I write I have something to drink. I am alone (most often) and have headphones on, which almost always play the same music.
I also don't consider my day is done until I wrote my words (there are a few exceptions, but not many). This is double-edged, however. At the start when I struggled to hit my wordcount goals, it was added stress. Now it stops me from sitting in front of a movie or video game or with a book upon returning home.
I know other writers take a day during weekends to do so. They put 7-8 hours straight into writing, blocking off all other activities. Whether or not any of these works probably depends a lot on what type of writer you are, but you ought to play around with it. I remember during the winter semester, though, I barely touched my projects. It's hard to focus when there's a lot going on, but if this job is going to stay in your life, you'll have to figure something out. But you have all my support! I'm sure you'll find a way.
