The best section of this great novel that can be used to answer this question is Chapter Twelve, which is when Lily finally has a long overdue chat with August about why she came and her past. Up until this stage in the novel, Lily has been struggling with hating herself for what she felt she did to her mother. As a result of having shot her mother, she feels she is not lovable and that everyone should hate her. August is able to make her see that this is not the case:
But you're not unlovable. Even if you did accidentally kill her, you are still the most dear, most lovable girl I know. Why, Rosaleen love you. May loved you. It doesn't take a wizard to see Zach loves you. And every one of the Daughters loves you.
Slowly, Lily begins to feel and to accept this love that is felt towards her by so many people. In addition, Lily also learns something very important about the human condition at the end of this chapter, after she has learnt about her mother leaving her. August tells her:
Every person on the face of the earth makes mistakes, Lily. Every last one. We're all so human. Your mother made a terrible mistake, but she tried to fix it... There is nothing perfect... There is only life.
Lily has to learn to forgive both herself, but also her mother for deserting her. August helps her to realise that all humans make mistakes, and we must forgive each other for the mistakes that we do make.