My posture in my chair is bad. My stomach sticks out and my upper back is pushed over the back (jetstream low.)

So far I have raised the back higher, but that did not help. The back angle is at 90* (I may have adjusted it to 85*) but I have tried all angles.

Cushion is a supracor stimulite contoured. I am not sure about the measurments of the chair, but I think it is 15" wide. I also think it is the wrong size. Could this cause bad posture?

I don't know where to get the measurements from because I did not order it and the chair is 3+ years old. It is a top end terminator titanium.