I have same issue because my pelvis is twisted. It's super annoying. I switched to a cushion that has pelvic support. Doesn't help a ton but some. I do have a corset ( much sexier than a back brace) and it helps but I can only wear it so long and still breathe.
What kind of cushion do you have on your chair? I use a ROHO Quadtro and have it adjusted so that the right side is more inflated than the left. That mostly compensates for my pelvic tilt.
"Freedom is what you do with what's been done to you." --J.P. Sartre