How to deal with a controlling mother?

I'm 21 years old and my mom still wants to treat me like her little child. I have my own job, pay for school and all my bills on my own, and help out around the house when I can. Yet still thinks that she is entitled to make decisions for me. And decisions about everything, from whom I'm friends with to how much money i spend on clothes. And any time that I disagree with what she says, she claims that I'm being "disrespectful" even if I am polite and logical about it. How do I deal with this? I'm sick of getting angry about this, because nothing seems to be improving.