My doctor says my cornea is too thick, causing my sensitivity to light. Why?

Corneal edema. The most common cause of corneal thickening (edema) with related light sensitivity is bullous keratopathy (here blisters form on thecornea due to swelling). Muro-128 can be used to improve symptoms. You should see an eye doctor for an evaluation.
Depends. This may be due to corneal edema or swelling causing irritation and resulting light sensitivity. This can have may causes and treatments including drops or corneal transplant depending on the etiology.