This doesn't really help you per
se, but I think that anybody who is breaking up with their significant other should be exactly aware of what they're doing. In the event that you don't already know this: you're probably going to crush her heart and soul. She's probably going to feel worse than death for a while. And you're going to have to live with that.
Having said that, if you don't want to be with her, this is the best route. Be direct, don't beat around the bush. Tell her straight, and whatever you do, don't say sorry if you don't mean it.