No matter what, during some point of a person's life, he / she is going to face a hardship, and they will have to overcome it. We have to stand up for ourselves to become strong.
Life is full of abuse, both physical and mental. "You keep pushing me nowhere" from the song I Don't Wanna refers to Celie's domestic abuse with her father. When Celie was young, she was rapped several times by her father. He even threatened to kill her if she told her mother. Since Celie is young, she doesn't know why she had a baby, and that is why she doesn't know who she is yet.
Mental abuse could also play a big role in a person's growing up. When Celie was young, and her mother died, Celie had to become a mother to Nettie. Celie was still young, but she had to take care of her younger sister. Celie was not ready to become a parent, and that was a big hardship in her life. "I don't know who I am" refers to Celie not knowing how to take care of Nettie. Celie is confused because she doesn't know why she is pregnant, and she is also worried about how Nettie grows up. Celie wants to protect Nettie from their father, so Celie took all of the physical, and mental abuse from him.
But later on in the book, Celie meets Shug Avery. Shug Avery took care of Celie, and she also taught her things like masturbation. This took some hardship away from Celie, because in a way, Celie now has a mother again, which is Shug. Also, in a way, Shug replaces Nettie. Celie love for Shug is different from Nettie though. Celie loves Nettie because she is her sister, and she wants to take care of her. Celie loves Shug for who she is, because she is strong, and was never abused. Although Celie has no proof that Nettie was dead, she still keeps Nettie in her mind.
When you know who you are, you can accomplish your goals. You have to stand up for yourself, and overcome your fears. When Celie was strong, she felt like she could do anything that she wanted to do. If Celie would put her mind to what she wants, she would accomplish it. When Celie learned how to masturbate with Shug, she learned about herself, and she was happy. "Just let me do what I can" refers to Celie being strong, and standing up for herself.
Another example of Celie becoming strong is when she says to God that her husband's name is Albert. In the beginning letters to God, she identified him as Mr. _____.
At that point in her life, she was still young, and un-educated. After Shug teaches Celie some things about herself, Celie becomes strong. She faces the fact that she was rapped, and then she says to God that her husband's name is Albert. She says this because she is strong, and she wants to fight back.
So in order to live a good and healthy life, you have to face your fears/hardships and you have to overcome them. Once you do this, then you are an individual; a strong, and healthy person that is free.