I'm exactly the same way and it's been irritating me since I can remember. I think the issue for me is being so convinced initially that the idea I'm executing

I'm exactly the same way and it's been irritating me since I can remember. I think the issue for me is being so convinced initially that the idea I'm executing is my ticket to billionaire-dom, that by the time a product is built and the real work is required, finding customers, reality sets in that maybe your idea isn't as amazing as you once thought, lowering my level enthusiasm and making me less willing to really put in the effort to see if the idea has a chance. I don't know how to fix it, but what has helped is waiting some time after I've come up with an idea to let whatever get-rich giddy emotions subside, allowing me to arrive at more realistic projections of how far an idea can be taken (with proper research of course). And like edw519 says, find a customer that is really interested in your idea. Having that validation from an already-acquired customer will probably give you the motivation to continue since your idea has been validated by someone other than you.