I'm going to take a cold shower now.
If you really want to take a cold shower fast, don't decrease the heat day by day. Do it in 30-second intervals. Within 5 minutes, the water should go from hot to cold and hopefully you'll have adapted fast enough.
Still, that's a little different from using mental strength. It's like the frog in the boiling water. I use the technique where the frog jumps into a pot at full boil without jumping out.