I hear you!!!!!!
I'm knackered constantly, and off to bed as soon as I can each night, anytime from 7.30!!! and I'm not working like you. My 1st pregnancy I was working fulltime and managing the labour only building of our first house, honest to God I don't know how I did it. When I was pregnant with my DD I had a 21 mth old giving me the runaround and even then I didn't feel as exhausted as I do this time (blame it on my age I guess LOL).
Takecare and rest as much as you can.
Cheers Missy