This used to happen to me sometimes. I feel like the issue is getting too excited for a new RP too quickly. You don't give yourself time to let the idea settle in your mind and decide whether or not it's something you can really keep going for an extended period of time.

What do I suggest? Wait [i]at least[/i] a couple days after getting an idea for an RP to actually start it. That way, if you were meant to lose excitement right away, it'll happen before you get other people invested in it. I also suggest that, in this time, you develop the idea as much as possible, which will either help you realize there's not much you can get out of the idea, or get you even more excited about it so that your drive to run the RP will last.