Carrie's "out" is quite simple, in theory. She felt pressured (not necessarily just by him, it was pressure from herself as well) into saying yes, then went away, and has had some time to think about her relationship and the past 5 months. And she just needs a break, to be by herself, 'find herself', enjoy her own company, and be away from people she feels somewhat suffocated by. She doesn't have to tit all over Joel if she doesn't want to - she can still just rationalise a break up in other ways.