I'm thinking of potentially breaking up a friendship of mine.
What makes you want to break up one of your friendships?
I find their reliance on me to be too clingy, and I need some space.
It is good to have healthy relationships and set boundaries with people. I don't think it's right to stop being friends with someone for that, though.
I just need some space.
Maybe you should just stay away from them for a little while instead of telling them that you no longer want to be friends. The time apart could make them realize that they don't need to rely on you so much, which would be great for both of you.