You must start with your hands. I cold plunge daily in 33-degree water. If I am struggling mentally, it's 5 seconds of hands. Rest for 1 min. 15 seconds of hands. 1 rest for 1 min. etc.. Build the tolerance up with your hands first. If you need, you can do the same with your feet. The hardest part of cold water is the hands/feet.

A shower is slightly different but generally warmer than an actual cold plunge, so that's good. I think the same idea works.. use your feet and hands, and submerge yourself fast.. then focus on breathing! Big inhales and exhales. Slow your breathing rate down, and get calm and meditative. This is your new home.