I have have had major insomnia throughout this pregnancy. At about your stage I went to the dr's and begged for something as I was hysterical with exhaustion, and they did proscribe me sleeping tablets, Zopiclone. I took those for a little bit and they helped.
Then it returned with a vengence a few weeks ago, I was awake from 3-6 every night. Went back to the dr's, and they reluctantly gave me more Zopiclone but were keen for me to take them sparingly as I am nearing due date. However I took one and it didn't help so having taken any more.
Just by chance I came up with something that helped. Usually I had no problem getting to sleep, just that I would wake up and not get back to sleep. But one night I couldn't get to sleep at all so I read and instead of going to bed at 9.30 I didn't go to sleep till about 12.20-1am, and that was the first night I didn't have 3am waking, I mean I still woke up to toss and turn but managed to get back to sleep.
I then watched a sleep programme the other night on BBC and it effectively said the same thing for insomnia, limit the time you are in bed, go to bed later and get up earlier, eventually you break the habit. However I would still try and sleep or rest when you can during the day as pregnancy is a bit different.
It's really awful and I do sympathise. Try going to your dr's and telling them how desperate you are. Another thing that helped a little was co-codamol that the dr proscribed me as I was so uncomfortable at night and that was another thing keeping me awake.
Are you a SAHM? I was working till 2 weeks ago but my work were really good and let me reduce my hours without reducing my pay. That helped a lot. But not so easy when you have other DC around. Remember TV is your friend! Do whatever you can do to get through the day. Hopefully your DP will let you have as much rest as possible.
Big for you as I really sympathise, and everyone just says it's normal, and 'it's your body preparing you for having a baby' which makes you want to punch them!