I am claustrophobic and can sleep in my cabover bed fine. The trick is that I sleep side-to-side on the outside edge not in the middle of the bed. I only tuck the covers in at the bottom and the window edge, so that I can stick my legs out over the edge of the bed and don't get caught up in the bedding. The direction I face is with my head on the driver side because I normally sleep on my right side - this lets me fall to sleep and wake up facing the larger space in the RV instead of a solid wall/window.
When I was little, we had a truck camper and I slept in one of those cupboard bunks with my older sister. She always took the outside, so I was literally trapped in the bunk. There was a very small window (maybe 1x1.5 feet) and I used to open it and smush my face against the screen to get the fresh air and relax. If your side windows open, you might try opening the one at your head. The overhead hatch being open/closed doesn't seem to make a difference for me.
You might also try disguising the dark paneling if you have it. Paint it white, cream or pale yellow. Eliminate the blinds on the side windows - no one can see anything from outside when you're sleeping. If the ceiling is not white, paint it now - that's a must. Get light or bright cheery colored bedding, it can help distract your mind from the low ceiling.
I suspect you don't have the front window - I find that helps a lot because it brings light into the cabover in the mornings when I wake. You might look for an LED string light with a timer that you can run along the front wall. Have it set to light up in the morning right before your normal waking time. That way you'll wake up to more light up there.
Good luck. Just keep at it - if you were able to sleep up there for the most part and just felt the claustrophobia when you wake, then you likely just need time to adjust to it for the most part.
