"A good plan is the secret weapon for combating mental obstacles. When you have a plan and are determined to stick to it, it doesn't matter what you are feeling at a given time because you can simply ‘proceed as planned’. Conversely, if you are plagued with negative thoughts and a lack direction, it’s easy to become complacent." 

forbes.com

The writer of this article puts it so simply, but it's really not simple at all. 

- image from lifeteen.com via google images.