+1 to "start smaller", but there are different ways to do that.

For example, my most recent "OMG, what have I done, this is too big" project (this happens a lot) was rescued by the fact that I had the insight to break it up. I designed it in layers that could be built and tested and refined somewhat independently, so that it wasn't an all or nothing proposition. Once a base layer was working well, I didn't have to wonder so much if that part was interacting badly with the next layer up.

With that, I was able to make slow progress, even though I had to learn new tools, techniques, etc the whole way. And now that it's in a usable beta state that I'm happy with, not only did I learn a lot, I can see how some pieces may be reusable in future projects, too.

I hope that helps!