i dont know well.
but tis is my think. = I don't know well, but this is what I think.

doctor is many but nurse is not many.
so my third dream is nurse. = Doctors are many, but nurses are not, therefore, my third dream is to become a nurse

and my mom want to nurse.
but nurse is very diffecurt. = My mom dreamed to be a nurse, too, but it's very difficult.