Like you, when I was prego with DD#1 I was working long hours (and in my case, visiting my mom in the hospital afterwards as she was getting chemo for leukemia). So getting "extra rest" wasn't an option for me either :(
Here are some things I did that seemed to help:
-Eat frequently. I had GD by this point with DD#1 and the GD "diet" required me to eat a lot of smaller meals. That actually really helped with my energy because I would always be mixing a protein (cheese, nuts, etc.) with something else (like dairy--yogurts, a glass of chocolate milk (i had sugarfree syrup :) etc.) I also drank a lot of smoothies for my snacks because I could dump in protein powder or peanut butter and make a shake with fruit, milk & protein or sugarfree ice cream & peanut butter. It helped keep my energy level up because I wasn't ever eating a big meal--I really wasn't allowed--but eating a lot throughout the day.
-Drink a lot. Mostly for me it was water (see GD) but while it kept me running to the bathroom it also kept me running and not dragging.
-I was (and am again) anemic. Taking the iron supps really helped with my energy, too. Ask your doc if you are anemic or borderline anemic and/or if you could/should take an iron supp for the energy. It certainly seems to help me.
Good luck and hang in there!