Sounds like fear of the future to me; you get a picture in your imagination going and then can't live up to that, still in your imagination, so "give up" all without doing much on the actual desire?

If you want to act, whether or not you become a "famous" actor isn't going to be in your control. I think you know that, know you can't control the future either and what may/may not happen. We only have "now". But you can't act in the future unless you work on it now and there's plenty of preliminary work that has to be done now before it is even possible to have that future goal! You don't have to be famous right this minute, you just have to take a single acting class or go to a single audition (don't even have to be hired for "that" particular show, either; it's like jobs, you may have to go on lots of interviews before you find one that fits you).

Make the list of shorter term activities that have to be done before you can become rich and famous and get them done before you worry about being rich and famous?

This book might help: http://wishcraft.com/