I don't work in the shower.
The detail that is important is not the knob, but the endurance of a blast of cold water.
I believe that you have missed my point. Turning a knob, over which you have complete control, really isn't subjecting yourself to anything meaningful. You're not pushing your limits. You're doing silly things and calling it personal progress.
Go climb a mountain. Or even a challenging rock. Commit to a period of time living in a developing nation, or any place drastically different from your home. Join the Navy SEALs. Do not, however, turn the shower tap to 'cold' and then blog about how you're growing as a person as a result of it.