Hello, Kristen1991, and welcome to Psych Central!
There's no easy way to communicate such a message to parents like yours. I assume you are an only child? It sounds like your mom has wrapped her life around you and doesn't want to let go.
What do you want out of your life?
A letter would be easiest, since they couldn't argue face to face against what all you say. But please don't expect things to go over well. They will likely still be mad, say they are hurt and perhaps even that you are a thankless child, etc.
Parents need to be able to let go--and I think healthy parents do.
Are you able to arrange to see a counselor about these matters? With the help of a counselor, I was able to write a letter and make more of an emotional break. I was very nice in the letter, but my mom still thought it was a terrible. Mind you, she and my dad had been writing me truly terrible letters for her years and years--threatening to cut me out of their will if I didn't do what they said, saying mean things about my dear husband, etc.
In my experience, my mom tried to take over my child. We all have fared well by living across the state from my parents. (BTW, my parents evidently both had personality disorders.)