I think that's pretty normal. Your idea could be about anything but it'll be the best one you've ever had and it makes your insides fizz just to think about it. It scampers around the back garden of your mind, squealing from time to time and demanding to be let around the front.
So you open the gate.... only to find that your idea has suddenly gone all shy, lost its self-confidence, is skulking in the dark alley and won't come out. Or worse! It appears in your public space in a blaze of noise and fury and then when it's there, centre-lawn so to speak, loses its nerve, like it has suddenly realised that it's dressed in a bikini when everyone else is wearing anoraks.
I'm like that. I have ideas all the time - mainly about how I'm going to become a successful writer, which is my dream. It's a lovely dream. Every day I take it out, huff on it a bit, polish it with my sleeve. But it's not very stable. It shifts about all the time. Just when I think I have hold of it, it wriggles out of my grasp.
And, I have realised, the slipperiness of my dream (and so the validity of my ideas) depends entirely on my levels of self-belief on any given day.
Yesterday : I am great. I can do this. People tell me all the time that they like my stuff. All I have to do is keep working hard.
Today : I don't know why you're wasting your time. It's almost impossible to get an agent. The market for the stuff you write is already flooded and anyway, what makes you think it's any good?
This is normal, I know. Head versus heart. The intellect beating the intuition into a pulp. But what I MUST remember is that how I feel on any given day doesn't alter the validity of the idea. If it was a good idea when I felt positive about it then it probably still is.
This is a picture of the sky over my home town. That's my idea, that is. That orange bit burning bright in the middle of all the grey...