So am in ridiculous situation, entirely my fault, engaged to someone I don't very much like. Ideally would tell them the whole thing was a crazy impulse, let them rant and rave a bit and then get back to my own life. But I'm too chicken and also can't bear the thought of how humiliating it would be for him (I don't think it's been two weeks since the happy announcement). So maybe it can fizzle out over a month or so, and maybe by then even he will notice that the happier he becomes the more unhappy and trapped I feel.
I really think he needs to be convinced it was his idea, doesn't handle rejection (or bizarre perceived rejection such as me wanting to visit a friend without him) very well at all. Quick and clean would be great normally but phased and uneventful will do nicely.