money is the other piece
I've found that GTD solves most of the issues that keep me awake at night, but it can't balance my budget if I don't know how to manage my money. Getting Things Done and Managing Your Money are those two high school courses that don't exist but should.
You have to have a firm grasp on what your work is, and how to save for and pay for everything you need to balance your life. Once you have both of those pieces in control in your life, there's really nothing you won't be able to handle... in my opinion.
So, although this isn't a financial advice forum, it's the puzzle piece that, when not managed properly, or at all, causes the most damage over time, regardless of how well you're executing your other projects and next actions.
constant forward pressure