I've never really known an ex-brony, but then again, I've only recently started to try participating in the community, so I can't really say much on that.
As for the second half, sometimes you just move on. I've lost interest in shows before, usually because they bored me or I didn't like the direction it was headed in. There's nothing wrong with it. Change happens. You can try to come back to it if you feel like you need a break, but if it feels like you've just lost interest in MLP, then there's no point forcing yourself into it. Do what makes you happy.