I feel your pain! I'm almost 38 weeks and found out today at my appt that I'm measuring 42 weeks! Last week I was measuring 37 weeks. I am so uncomfortable.
I have had sciatic issues before - NOT FUN. Make sure you are standing up often and not doing a lot of sitting. Sitting can make it much worse. But I know that is hard to do when you are so tired all the time!
Go see a chiropractor. I've heard the Webster Technique is good, and I think it's especially made for pregnant women. That is the technique someone suggested to me when I was having sciatic nerve issues (carries over into labor for me - awful).
For energy, I wonder if you're anemic. I get anemic at that time too. Floradix is a really good supplement for iron. And, I started eating at on of fruit around 32-33 weeks, and it gave me a lot more energy.
That all being said, I'm so exhausted all the time now that I have no energy...it's tough being prego!
oh, and it sounds like your baby dropped. Often it happens the most with first babies and not necessarily again in other pregnancies. But my 5th baby (current pregnancy), the baby has dropped. It really makes it more uncomfortable! I feel saddle sore on the inside. And it's almost like trying to walk with something stuck between your legs.
Hang in there. I have no advice really - just lots of fruit (seriously), find someone trained in the Webster Technique...and just try and survive. I feel like I'm in survival mode right now too! :-)
Good luck.