I'm not sure but I think that affected my life in other areas too. Although I read a lot of self-help, psychology, new agey, etc books in my 20s, no one thing helped as much as that. I guess it's part of learning to love yourself. I was trained as a kid and young adult to treat others well, but never to treat myself well! The idea of yourself being one of these people you should treat well, never appeared. You can be so mean to yourself, say truly horrible things you'd never say to a friend or loved one, and hardly notice, if it's something you've always done. And I was bullied a lot at home and school, so treating me badly was something it was easy to also do myself, until I learned just to stop, to never do it. Then when you are kind to yourself, it's natural to be kind to others.
Some books I remember helping with this: New Guide to Rational Living by Albert Ellis (on becoming aware of your habitual thoughts and self-talk, and changing them for the better where necessary), Wayne Dyer's first two books Your Erroneous Zones and Pulling Your Own Strings (similar to Ellis), Louise Hay on loving yourself, SARK's first 8 books, everything by Robert Fulghum, A Life of One's Own and An Experiment in Leisure by Joanna Field/Marion Milner (psychological self-explorations). Emerson's essays. Also keeping a diary was very useful for seeing patterns, e.g. I'd describe some emotion for what seemed the first time, then see I'd described the same thing 10 times before!
From the preface (or maybe chapter 1, I forget and gave away my copy so I can't check):
Modernization has always been a painful process. People
feel alienated and lost when fundamental changes in
their society make the world strange and unrecognizable.
... Fundamentalists feel that they are battling against
forces that threaten their most sacred values. ... Those
of us — myself included — who relish the freedoms and
achievements of modernity find it hard to comprehend the
distress these cause religious fundamentalists. Yet
modernization is often experienced not as a liberation
but as an aggressive assault.
https://en.wikipedia.org/wiki/The_Battle_for_God
Years ago, I read Speed of Trust which has made me a bit more trusting about the motives of people, so I don't default as much to the negative side of thinking people have bad motives / lack intelligence.
After reading about psychological safety in blogs, I decided to read The 4 Stages of Psychological Safety which I think will be very beneficial in helping me create a productive team at work.