Don't panic. Your walls are not square and that results in a gap at the bottom of the shower door. Rather than tear everything down and shim the drywall, just shim the door jamb. The bottom of the door needs to come in a little. So the bottom track needs to be shortened just a bit, and the bottom of the track shimmed inward. The gap between the wall or tile and the shimmed bottom of the jamb can be filled with caulk. The caulk bead will be a little wider at the bottom than at the top. Chances are, your walls were fairly straight, but you ran drywall over the tub flange. This caused an outward curve just above the tub. This would account for about 1/8 inch of the difference.
What surfaces are on the wall? Maybe we can offer some other solutions, but the simplest way to fix, is just to adjust the jamb and make up the difference with caulk.