It will soften the blow if you tell her positive things about herself, what you like about her, what was awesome when you where together and that this hurts you to do. You like her obviously, you just don't want to spend the rest of your life with her. Both of you need some more experiences before you know what you do and don't want in a life partner, she was there for some of the discovery part. Take what you learned together and remember it, remember her and how this felt to do.
Your very empathetic, don't lose that.