The core concept of Getting Things Done is built around the notion of a clear mind. The primary way to do this is to make absolutely sure you’re not using your mind as a storage for things you need to take care of. David Allen says: “if it is on your mind, your mind isn’t clear”. And a clear mind is what we want, because with it we can fully focus at the task at hand.
Getting Things Done means using something else than your mind as a system you trust for the full 100%. This can be a list on a piece of paper or a full blown task manager on your computer or phone, the only requirement is that you capture anything that bothers you. Not only related to your work: anything that needs your attention in your life needs to be there.
Want more room to think about the difficult things? Have an external system where you can safely deposit all of the things you have been trying to keep track of in your head.