How to do the right thing?

I’m ready to start something massive,something genuinely crazy. But I can’t seem to find the "where." With every idea I see, hear, or dream up, I can instantly find ten truly valid reasons why it’s destined to fail. I know business isn't purely a rational game and that you have to trust your gut sometimes, but my gut is a permanent skeptic; it either tells me "this leads to nothing" or "this is too small." How do you finally pull the trigger on the right thing? How do you know which instinct to trust? Should I just open a consultancy and start roasting other people’s ideas? (actually, never mind,that’s not a $20M+ business :/).