Most of the changes I've made are stylistic. It sounds better to keep mostly in past tense - switching around can be a little confusing, so I've altered where I think it should stay in past.

Christopher's mother

The letter Christopher's mother wrote to him shows the reason she left Christopher is was because she cannot could not bear the family. She is a natural person and everything she did followed the ways of her mind. Christopher's mom does not give herself many rules like Christopher. She just did does anything she wants to do. When she found that she could not bear Christopher's father and something that Christopher did, she did not stay with the family that she did not love it anymore and went away. (you could simply say she, 'she left the family that she did not love anymore' - it sounds smoother) She cares about Christopher and felt worried for her son, but these factors were not more important than the life she wants  wanted. She had the ability to decide make a choice, and tried to solve all the problems that this choice give  presented to her. She is was a brave women. In her mind, freedom and happiness is what she actually needs.
However, in other ways, Christopher's mother is not a good mother. When some problems happened occurred in her family-life, and her relationships with others broke, she chose to escape. She did not care about her son and her husband's feelings; she just left. She did not have the courage to face and solve problems with her family.
Christopher's mother is not a good person or bad person; because  (I would simply use a semi-colon here instead) peoples' minds in hard are complex, and people one cannot use the terms 'good' or 'bad' to describe a human being. Christopher's mother is just a normal person. To face When facing a problem in her life, she just simply made a choice, and her choice is was also not neither right nor wrong, good nor bad. She is not a good mother, but she is also not a good or bad person.