I think if you use the words " I feel" when you're saying why you're ending it, it can help. For example instead of saying " You treat me badly", you could say " Look, this is how I feel right now. I feel you treat me badly." Subtle difference, but a big one. No one can deny you your feelings or argue with them. They're your feelings and that's it.

But if you say " you treat me badly", it opens the door for disagreement when you state something as fact. You'd be amazed at how much it can lower the tension level because the other person doesn't feel that they're "on the stand" so much because it doesn't sound quite so accusatory. However it plays out, ending any kind of relationship is never easy.