A book summary from Fight Mediocrity (a youtube channel).
Big Idea I: If you are distressed by anything external, the pain is not due to the thing itself but to your estimate of it and this you have the power to revoke at any moment.
Big Idea II: You have power over your mind – not outside events. Realize this, and you will find strength.
Big Idea III: When another blames you or hates you, or people voice similar criticisms, go to their souls, penetrate inside and see what sort of people they are. You will realize that there is no need to be racked with anxiety that they should hold any particular opinion about you.
Big Idea IV: How much time he gains who does not look to see what his neighbour says or does or thinks, but only at what he does himself to make it just and holy.