Charlie states that he will "fight it" What does this statement illustrate about this character?

In context, this quote speaks to Charlie's determination.... his wish not to revert back to the "old Charlie", to remain what he has become.

Why can’t I remember? I’ve got to fight. I lie in bed for days and I don’t know who or where I am. Then it all comes back to me in a flash. Fugues of amnesia. Symptoms of senility—second childhood. I can watch them coming on. It’s so cruelly logical.
I learned so much and so fast. Now my mind is deteriorating rapidly. I won’t let it happen. I’ll fight it. I can’t help thinking of the boy in the restaurant, the blank expression, the silly smile, the people laughing at him. No—please—not that again . . .