Tell her the truth.
"I don't want to be with you anymore."
Most people ask for a reason to get closure, but emotions aren't logical. So just let them vent. Be classy, be kind, and be clear that you're done. You get what you want and treated someone respectfully even though it didn't meet their wants, Win/Win.
No wishy washy stuff. That just confuses the person receiving the bad news. It also sabotages your own real wants, Lose/Lose.