Time
Joe said it all. However I'd like to add one: continuïty in your time-in-shop.
You defenitely have to make a contract with yourself. And with your loved ones, and with anyone you know. One, two or even three evenings/days you will be in your shop. Working, dusting,thinking, be happy, be frustrated, be THERE.
On vast days. Only reason you should not be is serious illness.
Visitors are welcome - but on appointment only, and NOT ON THOSE DAYS.
Visitors ask for care, and care takes time. You want quality care for the one that bothers to look for your project, so plan it on a day that you can give all care needed.
In the building part you will undoubtedly suffer for some frustrated moments. Afterwards you will laugh at them, but in the valley all hills seem untamable. There are many, many 'projects' for sale in this world: kits that for many reasons never get the state of a flying plane. Don't let it happen to you! One way to handle that risk is continuity in time.
Even if things don't go the way you want, even when your out of ideas for that particular problem, even when it is cold in winter and your heater doesn't work - be there. Be part of the project. Become it work for you!
And my last tip: don't build an airplane.
Do not be too eager to get things done. This project will certainly take a few years of your life (and that of your loved ones), enjoy! Just build one project by the time. Prepare the project, prepare the things you need, prepare the shop and get that single project done. And yes, one day it will become part of an airplane- or maybe not. But in the meantime it gave you the satisfaction that building an airplane is all about.
I can tell, I build a KitFox on my own - without any other experience in building than model airplanes.
http://kitfoxclassiciv.blogspot.nl
Now I build an RV12 with a buddy. For me that means: More security in finance, in building,in fun.
But we still build one evening per week - every week.
Take care,and enjoy!!!