One of the mental game classics for sport was a book written by Dr. Bob Rotella called 'Golf Is NOT A Game Of Perfect'. 
It was one of the first books that really helped me to understand that mistakes happen, when they do it's not a disaster, and it doesn't make you less worthy.
BUT... how you respond and recover from those mistakes matters. That's what illuminates your true character.
There's a great saying, "sport doesn't develop character, it reveals it".
It's easy to look strong when everything is going well, but not so easy when things are going wrong.
If we start to accept that there will be good times and there will be times that are trying, we can start to appreciate the good times more as the gifts they are, and understand that the times that are more trying are going to happen anyway ... so we may as well take the lessons that they offer, versus wishing our lives away for some future joy—simply because we are facing...