I am sort of in the same position as you, feel exactly the same physically, but have sort of come to terms with it emotionally (helped that scan showed everything perfect...just need to find something else to worry about now!)
I hated everyone telling me that they had marvallous pregnancies, in that very smug way they have when faced with some who is suffering.
Take the time you need. I have just had 4 weeks off (1 working from home, 3 sick) due to this and also with a bad back. I feel refreshed and rejuvenated, but I know if I had tried to do it all normally that I would have ended feeling even worse.
I am in a sort of lucky position with regards to work as well. I have just been made redundant, although financially I am going to be OK, unlike some other people who are not so lucky, I know, but it does mean that I can spend the last few months before my baby is due taking that time out for me.
I had planned on an early maternity though, before all this came up, I couldn't bear the thought of being fat and huge during July and August, my SIL suffered last year and there was no way I was going through that. People looked at me as though I was mad! I was told I would get bored and lots of other horrible things they say when you tell them you don't plan on doing things exactly like they did.
Stand your ground and let everyone know that actually, you are important, and yes life does go on, but right now the priority is for a happy healthy mummy and baby, and if that means changes so be it.
I have been lucky with quite a sympathetic husband, and my mum has been great, but I have still had to remind them every now and again, that hang on, I'm struggling here and I need some help.
Chin up, you are not the only going through it, and ignore all the absolute pains in the backside who tell you you can't cope, tell em all that if they persist in saying things like that to you that they won't get the chance to see you prove them wrong. Grrh!
:-)