What if your sleep is broken up???

My sleep is pretty crappy. I was taking 7.5 Mirtazipine for about 2 years and it worked and then started wearing off. I stopped taking it for a month and only took melatonin. My sleep was about 3 on and 3 off. THen I got back to the Mirtazipine and I slept great for one night. Then the next night it was pretty weak, may 5 hours. Then the next night 2-3 hours. So I figured I'd stop taking Mirtaz so my body could build up sensitivity to it again. So only with the melatonin I was getting 3 hours. AFter a couple days I took the mirtazipine again and again I only got 3 hours..with both the melatonin and mirtazipine. I finally went to the doctor who gave me hydroxyzine but it didn't work. THey put me on the lowest dose possible. A new doctor then gave me Mirtazipine again and told me to take 7.5 for 7 days and then switch to 15 mg after that and come back in two weeks. Well, now I'm on 15mg and my sleep seems to be 1 hour of sleep, then wake up, pee, fall back to sleep for 2 hours, wake up for abt 10 minutes, then sleep again for 2 - 3 hours, and then that's the night. What do you think is going on? I should mention that yes, I have VERY SEVERE PTSD from past abuse and bullying.