I'm far from an expert! I've just picked up some knowledge from trying to fix myself and other people I train with. Feel free to try Scon's (or anyone else's) suggestion, but you're likely to find that until you loosen some other stuff up you'll be pulled back in bad positioning. I have a different exercise called the crucifix so this might get a little confusing
Can you remind me what equipment you have access to in terms of bands and balls/rollers?
No worries
I just tried scons crucifix, and yeah, I can do it, but the main limitation is (surprise surprise) in the shoulders...Weirdly my left one.
With regards to equipment, I have nothing, the only thing I have is a doggy ball which is solid rubber, so has next to no 'give'. Main reason I got it was to free up my scapula as it gets knots all the time