You've already gotten some great advice here but I wanted to chime in with a few tips that I have used in the past.
After a very stressful day, get into your workout clothes. Start with a few minutes of quiet music, stretch, relaxation, breathing, etc. Give yourself just a bit a relax and veg out.
I always do at least the first fifteen minutes of my workout no matter what happens or what my motivation level is. You can get yourself to do anything for fifteen minutes. By then, I am usually into the workout, more motivated and in a better mood. If I am not and I am really pulling to stop the workout, sometimes I do. If you are fifteen minutes in and you still are dreading it enoug to press "stop," then maybe your body is telling you something.
You may also consider a different workout schedule. Shorter workouts are sometimes easier to get through, quicker to motivate, etc. Every other day might be all that you can do right now. Try different things and see what works. Push yourself but don't push yourself beyond your limits.
When I am really lacking motivation or energy, I turn to my "favorites" and only my favorites. You are more likely to do the workout if you like it and you are most likely to stay motivated and give it that little extra that you might not have if you knwo that you are going to have a good wokrout in the end.
Turn the music up & elevate your mood
Don't be afraid to rest if you need it. Sometimes you really do need rest more than anything.
Adjust your diet to have good foods, healthy choices, good water intake, etc. This helps when energy is low (and stress is high!). It takes extra time to plan, etc., but in the grand scheme, it will help to spend a couple extra hours per week planning and packing.
Good Luck,
Christine