What I got from GTD is to extract everything you need to get done from your head into one Inbox from which to process those things.
Specifically, to make the time and space for it. Every todo item that is sapping your energy and thoughts that comes out lightens the load.
This needs an uncomfortably long time and the discipline to not start doing anything that comes to mind.
Write the thoughts down. Sit still for longer than you want. And afterwards, keep on capturing anything else that comes up immediately.