How to remove protective film from shower after being left in the sun
No doubt the adhesive (and maybe the sheeting itself) has permanently hardened at room temperature. You'll need to either warm it or use a solvent to soften the adhesive. 

First, I'd try a heat lamp. An infrared flood light would do, or a halogen utility light in a pinch. Set it up so that it's no closer than maybe 24" from the tub at any point--you don't want to scorch the surface. Monitor it closely and often. Work one area at a time. 

If that doesn't work, move to mild solvents like GooGone, depending on what the manufacturer says about cleaning solutions.