I have had trouble in the past sitting for long periods (now I can't sit
for other reasons). What made the difference was putting my feet up on a
foot rest or a slanted foot rest. Like you, a pillow behind my back did
nothing. The main thing for me was to change the position of the lower
back so that it's not too curved/bowed but more flat.
Pain wakes me up, too, although it was a lot worse right after I got TM.
Now, I can usually just roll over on the other side (after a trip to the
bathroom) and go back to sleep. Between waking up because of pain and
having to empty my bladder, I'm a lot better at going back to sleep than I
used to be--I purposely try not to wake up too much by not checking the
time or doing anything that makes me think too much. You might try
propping yourself in different positions, with pillows if needed, to see if
that helps. After my surgery last summer, propping myself with pillows was
the only way I could get comfortable enough to sleep, even with morphine.
When I was in rehab for TM, I was encouraged to change positions several
times during the night and used pillows to put myself in different
positions.
Hope you find some relief.
Aurore