I'm going to tell my mother to stay out of my finances
Why do you want her to stay out of your finances?
As an 18 year old I think it's time to take the control over my finances
Even though she just wants to help, it is okay to want to do it yourself. You need to learn sometime and it is better to learn when your finances are not that complicated.
Exactly, I can do it on my own.
I'm sure she'll understand when you tell her you are mature enough to start doing them on your own.