I can’t give you a definite answer about why this is happening on the basis of a letter. I can tell you that it’s not unusual for a teen to question herself. Since you are hearing your own voice, not someone or something else’s, it may be that you are being very hard on yourself, not mentally ill.
I do hope that you aren’t diagnosing yourself as having depression or DID. Self-diagnosis is often misleading or just plain wrong. You need a qualified mental health counselor to make a diagnosis and to answer your question.
The low self-esteem is something you can work on — starting right away. The key to having a positive sense of self is to get out of yourself and start doing positive things for others. Find an activity with other people your age that makes a difference. Perhaps there is something you can do where you can put your artistic interests to work. If your school puts on plays, for example, you could get involved with set design. Or you could volunteer at a Saturday art program for younger kids. If you are stumped for ideas, talk to your school counselor or advisor.
I wish you well.