For me, I "knew " it long before I ever really "got it".... it's not a rational thing that you can learn in a book, it's like the others are saying - more about putting it into practice in ways like stopping negative self talk and starting positive replacements for that chatter.
I also found it helpful to look back over my perceived "failures" & examine them again with a bit of distance. What happened as a result of my "mistake "? Did the earth stop spinning? Did they take away my birthday? Did anyone suffer irreparable damage? If other people were involved/present, did they even have a clue that I was even struggling with it all, or do I only perceive that they notice my every little action/reaction? It's difficult but incredibly beneficial to see yourself clearly without running it through the filter of self judgement.