A very long time ago at the sadly losing since passed Paradise Lost in NYC I had a very intense CP session with two Dommes that lasted for two hours: several canes were broken over the course of that time. In order to keep me going one of the Mistresses made me stare in turn at each of the paintings on the walls and then made me call out the names of the different colors in each of the paintings. It had the same effect as you’re looking for with a mantra and I was very grateful that I stayed the course.