I happened upon a technique that works. If you have to do something big, don't try to deconstruct it to smaller tasks, plan it all out and create a schedule. Build a prototype of it instead, then a better prototype and so on. If you must have a deadline, have a close one, like next week or so.

Winning is really important, because the human brain is really short sighted, it needs to see that things are progressing, and it needs to believe that something can be done.

I'm working on an app that let's you measure your progress using a conversational interface that also suggests things for you to do. It purposefully only allows short-term planning, and supports no planning at all. It learns of your life through conversation and gives you tools to help change it. This is kind of what a psychologist does, they reflect you back to yourself and encourage you to try things, which works better if you can succeed at those things and believe that you can.