I've had that in one car and on the pillion of the bike. In both cases it was the BACK, not the seat. The back was too deep making the seat too long. Try putting a pillow/cushion behind your back. If that does fix the seat, look for a good solution for the back.
I do have a cushion I bought at a back store in my truck. Only because the seat was worn out by a heavier person and left a good depression there. Needs to be repadded. The cushion is wedge shaped.
I tend to sit on one foot at the computer, and simple sewing and embroidery. I can't use a chair with arms.