You understand Bayes' Theorem. You enter into a situation with an intuitive "common sense" prior. You observe the situation, and then you shut up and multiply.
And then you go to update, you compute the desired behavior to maximize utility... and some cognitive module buried in your brain says "no".
Example:
I realize I need a physical examination. I have no rational reason to fear going to the doctor. I am, in fact, acutely aware that my fear of going to the doctor is based on a fear that they will find something wrong that I can't afford to fix, but the truth is already so. So I bite the bullet, make an appointment, and then at the scheduled time I get in the car and drive to the doctor's office.
And then I just keep driving past the doctor's office, turn around and go home.
I tell myself that if I'm not going, I should call the doctor to avoid a $100 no-show fee, but I don't.
And then I get home, and I tell myself that that was dumb, and that I need to update my behavior - and that physically punishing myself for not going to the doctor is not an efficient use of my energy.
So I punch my hand through a mirror.
NOW, finally, I have an excuse to go to the doctor - so I wrap my hand in bandages and go back to bed, instead.
What do you do when your computed probabilities and utility function have NO EFFECT WHATSOEVER on your actual behavior?
Cede the battle, heed the war. Learn to fail better. Vague comments, but some some cognitive module buried in your brain says "yes." Be okay with being dumb as a rock sometimes, maybe much of the time. Abandon the quest for why this is happening, because that's not helping you change the course of the behavior. Stop looking for the source of the river, and stop trying to dam it up. Redirect it. Maybe take a friend to his or her doctor's appointment as a 'dry run.' Or because it's a nice thing to do.
Example:
I worked at a needle exchange where IV drug users could put their needles in a sharps box instead of random trash cans or on the street, plus pick up some first aid and referrals. One guy would come in every now and then. He came in one time with a red nose. A few weeks later, a nose with a boil on it. A few weeks later, a big open sore on his nose. A few months later, the guy was walking around with a rotten hole where his nose used to be. In the 20th Century, in the United States, in a city with a social service offering him 100% free medical care on demand, he let his nose rot off. I'm very sure he's dead now. And he wasn't dumb, wasn't even below average, and wasn't crazy. Just got lost in his own thinking and put things off.
Don't be that guy, you big dummy. You blew it recently, try again later. Try new ways.