Retiling a tub surround, not sure how to shim it
I see that the floor tile (and possibly the plumbing, a cement bed, or whatever) prevent you from moving the tub. That was going to be my first suggestion. If it's a possibility, do that. Move it only as far as necessary toward the long wall to make it flush with the cement board.

Otherwise, I also see that you have the drywall cut back some distance on the flat wall. I'd cut it back further if you can, then intall the shims as you describe above the tub. Taper shims out on the studs outside the tub. Use a 4' or 6' level or other straightedge horizontally to find the best compromise between flat wall and parallel to the tub. (You may need to continue the taper over the tub, toward the corner, to maintain a flat wall.) Take the taper all the way to the next corner if you need to and can, or as far as possible. 

It appears that you only need to gain about 1/4" (I'd eyeball the gap plus the thickness of the tub flange to be about 3/4".) That should be no trouble to taper out.

Carpentry, especially in remodel situations, is often about finding the best compromise from an aesthetic standpoint. Here, because your floor tile is diagonal, you can afford to fudge the wall a bit to make all this work together.