Charlie has a continual internal battle with pre-operation Charlie. Why is this significant to the main themes of the story?

In Flowers for Algernon, Charlie Gordon's life is changed by an innovative new surgery that triples his intelligence. While the success of the surgery affords Charlie many new opportunities, he is still plagued by the difficulties of the past. Post-surgery Charlie has great trouble understanding how dim he had been and is embarrassed by the lack of understanding he used to have. Even as he reaches the peak of his intelligence, he is reminded of his former self by the busboy in a restaurant, and after an altercation, he leaves the diner in shame.
Charlie's struggle with his pre-operation self contributes to several themes within the novel. The first is that memories of the past play a role in the present. Even though Charlie's new intelligence presents him with an opportunity to leave the past behind him and begin anew, he never seems to be able to completely start over. Often, he recollects experiences or conversations with his mother that demonstrate how his past has shaped his personality. While he may be smarter than he used to be, this new version of Charlie cannot erase the experiences of the past.
Charlie's struggles also deal with the theme of self-acceptance. Charlie mentions many times that his goal is to be like everyone else; even in his life prior to the operation, Charlie knows and dislikes the fact that he is different. As the surgery takes effect and Charlie becomes more and more intelligent, he is mortified of the way he used to speak, write, and behave. Charlie's feelings about his former self, even when he is in a position to put that behind him, demonstrates that he is not happy with who he was, and that makes it difficult to be happy with who he is now.