Casual sex should be with somebody you actually like. My new rule of thumb is to only shag men I don't mind sleeping over, and am okay with kissing goodbye when they leave. Anyone else isn't worth my time- and if I'm not prepared to make at least that much effort, I'm probably not worth theirs, either.
The general assumption that 'time heals' makes people do nothing about their emotional pains. It validates the idea that prolonged anxiety or depression will eventually just go away. Even the language of 'moving on' implies that some kind of temporal 'passing' is required to get over something. This is simply not true.