Brains are sub-optimal
To a first-degree approximation, your brain looks at things around you, conditions on it, then traverses an associated linked list of thoughts, and turns thoughts into actions. It's a lot less glamorous than your self-image as an optimal problem-solving agent, but at least it's closer to truth.
The silver lining is that your thinking brain can use its self-knowledge of its own sub-optimality to become more optimal. Example useful strategy: use thinking brain to decide what you want to do less of and what you want to do more of. Then:
to do X less: remove X from sight, and/or increase the barrier to X. (e.g. do not buy/ display chocolate. put away snacking apps to page 5. hide the bookmarks bar)
to do X more: add items reminding you of X and/or reduce the barrier to X. (e.g. don't stow dumbells into a storage area, leave it out on the floor)
That's just one example. The general point is that you have to recognize that you are dumb, accept it, come to terms with it, and forgive yourself. Only then can you trick yourself into becoming smarter.
(via Andrej Karpathy)