The one true inescapable fact of exercise for strength and stamina is it very challenging. Fundamentally this is exactly why it works. On a physiological level the intense challenge stimulates your immune system to flip a biological switch.
Typically things that are tough but within your present ability are met by your immune system working to maintain homeostasis as in preserving your present day health and balance. To improve your present condition means you have to go beyond this at least temporarily.
This choice to force yourself past your regular state starts in the mind not in the muscles. It is a decision that must be followed through on to demand the adaptation switch be thrown by your immune system. It is this aspect which we think has a marked effect on ones sense of self and allows positive practice in strengthening ones sanity.
Continued in part 3: https://wefit.ca/2021/03/01/strength-and-sanity-part-3/