First of all, the whole kind of "no pain no gain" philosophy is particularly American. In a lot of other countries they find this mindset bizarre. If you go to the gym and it hurts, you should STOP. Maybe it will take you twice as long to build up muscles as the guy who "leans into the pain", but you're also not going to tear your hamstring, or slip a disc, or whatever, and you'll still achieve your goals.
As for "psychological pain", again, don't "lean into it". Figure out where it's coming from, and fix whatever is causing the pain. In the example of merges, use continuous integration, or merge daily. Which is what the author says... but that's not leaning into the pain, that's intelligently avoiding it!
And if selling is a painful process for you, then don't "lean into the pain" and keep doing it painfully -- find someone to teach you how to do it better, so it stops being painful.