Yikes. Looks like it's more than just her being annoying. The fact that you feel guilty really is more of a "bitch back the fuck up" concealed within your good intent. Look, your a good person. You wouldn't be if you just kept it inside and blew up about it. But you really need to be honest. Perhaps if this was something that occurred once in a blue moon it wouldn't be that big of a deal, but it sounds like it happens often, and you have to put up with her often. Be completely straight. What do you REALLY think about her....