In Flowers for Algernon how was knowing what the outcome would be difficult for Charlie?

As a mentally-deficient individual, Charlie was only slightly aware of how others viewed him. When he became intellectually capable, he recognized all of the condescension, pity, or covert ridicule that his condition had engendered. Even knowing that it would not matter much to his past or future selves, the "aware" Charlie was upset and despondent about losing his newfound identity.