In these situations, I think a white lie is the best way to go. I might say something like "Hey, sorry I've been a bit distant. I've started becoming more serious with someone in the past week. I had a lot of fun with you though. Wish you the best" It doesn't have to be that, but that's just the general gist. Basically a white lie that doesn't hurt anyone's feelings so she can get closure without feeling like shit.