My biggest downfall was constantly changing my mind. I have a very active imagination and am constantly thinking of new things to make or do.
Like many of us, life got in the way, am on the tail end of a divorce, and have to build a house first.
I've helped plenty of friends build cars and can do most things, things that kept me focused:
Make a plan
Get friends involved (Read accountability to someone helps)
Small projects and complete them

The main one for me has been know your limitations, and what you will compromise on. Also, what is important to you, focus on that.
For me, it's all about the engine and the mechanical things around it. My car, when I get back to it, will be about the drive line, good fabrication and lots of unnecessary billet parts! Because that makes me happy!