I am sorry to hear this!
Ah yes, those sympyoms are very similar to mine. Basically, it is inefficient sleep. Like you, I too wake up about every 20min to 1/2 hour. My sleep studies also all turned out negative, however it turns out that those studies were focused on breathing problems, etc. I am going to a third one soon, which will be given with an emphasis on neurology, who focuses more on what I think is going on.
The point is, check to see what your sleep studies focused on, and make sure they encompass as wide a range as possible.
I have not found a solution yet, so the only thing I can do is sleep much more. That is, "sleep" 9 hours per night, and then takea 1 hour _max_ nap in the afternoon, every day. Those naps make all the difference. Give sleeping more a try. 7 hours per night in itself is pretty little, not to mention that you are not actually getting restorative sleep.
Also, maybe you want to try taking Melatonin? Perhaps start with 1mg for a few days, maybe 30min to an hour before bed, and then work up in 1/2mg doses every few days until you find something that causes you to sleep a bit better, and then finetune the dosage--but not so high a dosage that you get night sweats, more too much dreaming, etc, which happens when too much Melatonin causes hormonal imbalance. More is not better
For some people, 10mg is too much, for others, 2.
Despite being subscribed 10 different sleeping meds, Melatonin is the only one that works at all, most nights. 1.3mg is what works for me.
I'm sorry I could not help you more. But, bottom line is, one has to sleep as much as the body needs, otherwise other things start breaking down; and it can take years to build back up. Listen to your system
Do you have sleep apnea? If you have breathing problems (sleep apnea, etc), then laying on your back will indeed make it worse. But, maybe a breathing machine would be better? They're costly, but insurances may cover it. Who knows? Maybe the tennis ball will work.