How to remain poised and graceful during a break up?

Simply respond with "You've done me a huge favor, because I now know that if we had stayed together, I would have continued to be miserable. Now at least I know I still have a chance to be happy, and am relieved that it won't be with you." After that, walk away, secure in the knowledge that everything you just told your douchebag ex-boyfriend is perfectly and completely true, and you've made it clear that it is all his fault in a very subtle way that he may be too stupid to notice.