I don't want my grandma to treat me like I'm a kid
Why don't you want her to treat you like a kid?
Because I'm an adult and can make my own decisions.
It's understandable that you want your grandma to treat you like an adult. I think you should try to show her that you're capable of making decisions for yourself, so she sees that you aren't a kid anymore.
She will never understand.
You know until you try. Some people may surprise you.