Seeing how many dressage saddles you've tried, I think experimenting with other styles would be a logical next step. Perhaps the less-open hip angle of an a/p or jump saddle would be more comfortable.

If you can't find an answer, you might want to consult Anne Howard www.in-balance.com. She's a physical therapist and a dressage/event rider, who has a real gift for fixing 'broken' riders.