Even though I should be asleep right now (been awake for 20 hours straight, 80 hour work week

) I have to say this:
You need to be honest and upfront with her. Trying to let her down easy isn't really a bad thing, but making sure that she understands what you feel, and why you feel that way (ie. long distance relationship) is what is key. Sometimes, breaking up is harder than it needs to be, for both of you, and that is just something that you can't do anything about. Have either of you tried to do something about closing the gap in the distance, like occasional weekend trips, or even moving closer to one another if possible? I am just trying to throw out some ideas to maybe save the relationship, as it seems like you really care about her a lot.