"
If you look in the mirror and don't like what you see
You can find out firsthand what it's like to be me
This is Gerard Way talking about depression and not liking yourself, and speaking from his own experience. "If you look in the mirror and don't like what you see" -This could be literal, but it could also refer to disliking other things about yourself other than physical appearance, such as intelligence or talent. "You can find out firsthand what it's like to be me" - This is Gerard saying that he went through or is going through the same thing, and you shouldn't just decide that his life is amazing because he is the lead singer of a band and has thousands of fans.