I really wish I had some super duper wise words, but if I were you I would write your mom a letter.
Tell her everything you want about really truly feeling like you need help. Tell her how you feel about her dismissing your feelings.
Then you can leave the letter for her to find, on the kitchen table, on a mirror....wherever you think she will find it. Do it before you leave for school so that she has the day to read it and think about it.
The reason I would do that is because she won't have a chance to stand there and argue with you or put you down or dismiss you. She will HAVE to sit with it for the day and then, hopefully, will realize that you are trying to be a bigger person, a mature person, one who knows what they need to be happy. Maybe at the end of the day she will come to you and just talk....not argue.
I really hope something great comes of your situation. I am really glad that you know what you need and want with life. Keep pushing her...maybe she'll see your efforts to improve your quality of life.
BJ