Sometimes you just gotta do it. Don't think, don't breathe, just take the plunge. If you feel excited about it, go for it. It's scary, you have no idea what's gonna happen or how it'll turn out. But guess what - you can always change your mind later. Nothing lasts forever, and sometimes the most sane thing to do is to kill your darlings. But that's later, in the future. Possibly.
For now, just go tell someone about whatever it is you want to do before you get the chance to back out.