I have frequently had the same problem, waking after 5-6 hours of sleep and unable to get back to sleep. I don't remember if this started when I was pregnant (I'm not pregnant now). I know I had some trouble sleeping later in pregnancy but that was mainly due to being uncomfortable. The trouble for me is that (I think) my mind won't settle when I awake in the middle of the night. Sometimes I wake because I need to go to the bathroom, then I can't get back to sleep. I have really tried to watch my fluid intake later in the day, and I try to drink a lot early on so I am getting enough, and that helps at least with that. I also tried keeping a pen and paper next to the bed so if there was something particular that my mind was dwelling on, I could write it down and hopefully leave it for later. That didn't really help me so much because the act of writing caused me to wake up to the point that I could not get back to sleep.
I don't know if I really know of a possible solution for you, I just know how agonizing it is! I'm thinking that a little exercise might help. If you can squeeze in a 30-45 minute walk at some point in the day, that may help you. And when I do wake in the night, I try to keep everything out of my mind. I really fight to not start thinking. (hard to do). In relaxation techniques, they tell you to focus on your breath. Sometimes that works, sometimes not. What has helped me the most is that if I feel myself waking, I try to fight it. That's hard to describe because I'm half-asleep at the time, but somehow it seems to work. Anything that I do that forces me to wake up (like going to the bathroom or writing) makes it harder to get back to sleep. So for you, I would suggest not getting out of bed too soon. I know how miserable it is to lay in bed forever when you can't sleep, but maybe if you try to get back to sleep without getting up, it may work for you (?)
I know I'm not much help, but I feel your pain and I hope you find a solution!