I know how you feel, and I'm sure many others here know the feeling too. Unfortunately there is just no good way to say "I'm sorry, I know we had a good friendship at one point but it's not there now and I don't want to keep in touch with you" without sounding like a cold-hearted b****. I'm guessing the guilt is from ignoring her, not from actually letting the friendship go. I've done the same, and hoped things would just fizzle out. It's a bit hard when people view a friendship differently.