There is no way to avoid this being horrible for the person in question, but sometimes things have to come to an end. Be 100% honest - from personal experience being on the recieving end, if you are dishonest when you do intitially break up it can make thing far more difficult for the other person when the truth inevitably comes out. There is no way to make this easy for the other person. Just leave them in a position where they know it's over, understand why (it's very, very important) and have enough closure on everything that they can, hopefully sooner rather than later, move on, and hopefully have happy memories of being with you. Don't try to make them hate you... that never works in the way it's intended to.
You may think that.
You might find they don't care quite as much as you obviously think they should....
Oh dear......They will just have to be devastated, huh?.......
I was devastated after a 2year relationship ended. I was absolutely powerless to stop it.
My suggestion is to, perhaps, remain friends. Let her know you still care, but can not be in a romantic relationship.I feel my ex could have been so much kinder, he fully broke my heart by his heartlessness, self-centredness and cold attitude.
I love Top Cat! Sorry, that has nothing to do with your question, but I haven't thought about that cartoon for decades.
Sad isn't it, that it's almost as hard to be the breaker-upper as the breakee. I have no words of wisdom for you on this one, but I appreciate that you want to make it as least painful as possible for her.