I've started to do the breathing technique recommended by Wim Hof. I do it lying down in bed to energize myself for the day.

Then I do pushups and crunches. Today I lost count while doing crunches, and I did an extra 10.

The breathing gives me strength and endurance to do these exercises.

He also recommends cold showers, starting out with 15 seconds of cold water at the end of a normal shower and gradually increasing the time. I just cannot do that, but I can splash cold water on my face, hair, forearms with no problem.