I myself could probably never become someone who does only one project at a time. It would require too much discipline. I guess it would be helpful to find something that really interests you. A couple of years ago I found I had way too many UFO"S etc. I made myself finish at least 2 projects to start one. I usually finished more than 2. I have 6 boxes I purchased at JoAnns. They hold most quilt project nicely. They are about 14"square. When they are all full, I won't let myself start anything until I have an empty box to store it in. I usually have a paper pice project going. These are usually something I don't always have the brain to work on all the time. They require too much thought for some times. I always have some sort of charity project going, usually an easy somewhat mindless project etc. Works for me. I know this isn't very helpful for you in the one project at a time question you asked. Maybe if you could limit yourself to less projects? Good luck.
I just don't have the discipline to only do one project.