I am a 12 year old pianist, cellist, and flutist.
I really love music and I want to be music major.
The thing is, my parents and my music teachers tell me it's not going to happen.
I compose a lot and I really enjoy it.
What should I do? :O

Very good.

good.

So .. I am a parent (a dad) with a 17 year old, a 14 and a 12 year old.
I am pretty sure your parents (and teachers) are keen to think about what is best for you.

When I was younger I dreamed of being a composer and a musician.
Now, I am not (just a proud parent - but that's pretty good), oh and a very keen amateur musician having a lot of fun playing in a jazz band, a rock band and with family and friends.

It's good to have dreams and ambitions - but be careful not to hang everything on your dreams - not everyone can be a top musician or a composer.

I would say it's too early to tell when you're only 12 years old. Keep an open mind - who knows what you will find in the future that inspires you. And you don't have to let go of your dreams either - just don't get too fixed to young on one track or one ambition.

Most of all - enjoy your music and share in your music making with others. it makes it more real and perhaps a little less intense.

I have always found when you worry less about things they turn out better.

its not necessary to being a musician for dealing with music. dont forget you can always continue and develop to playing this enstruments when you work in another job.
Success