The same way you'd break up with anyone else (even if someone HAS done something wrong, do you think they'll actually believe you're right about it?). Sit them down and gently tell them that although they're terrific, you just need to move on, you're not ready for something longterm yet, and you both want different things. Neil Strauss, in his online blog, recommends that you make yourself available by phone and text for your ex to speak to for the following two weeks, but avoid in-person contact entirely as it is very easy to get swallowed back into a relationship if you allow personal boundaries to be crossed. After that two-week grace period, cut off contact entirely until you have both long since moved on. And don't flaunt your other involvements -- if you start dating in the next month or two, keep it subtle. Of course you want to be as gentle with the other person's feelings as possible, but in the end you really don't need any reason to move on from a relationship other than that you don't want to be in it anymore. No one is ever obligated to be somewhere they don't want to be. Just be kind about it and then firmly stick to your decision.