As an experiment, I tried changing a life-long habit: from the time I carried a wallet, I always put it in my back left pocket. Did it without thinking. Always put my keys in my left front pocket (without thinking). I started putting my wallet in my right-back pocket, and my keys in my right-front pocket. It took a looong time to break myself of the habit. It was so easy to use the left pocket that I'd do it without thinking, and realize what I had done minutes later. After a while, it got easier, but I would still regress if I didn't pay attention. Now, putting the articles in my right pockets is reflex, but every once in a while I slip...
It has taken years to get to this point, and it was a learning experience: I'd read all the '30 day habit' books too. I guess it's easy to start a NEW habit, but changing an existing habit is hard work.
HTH