"Once you know with absolute certainty that nothing can upset you except your own imagination, you will succeed in disregarding your desires and fears, your worries and ideas, and live in truth alone".
It's from the book "I am That", which contains conversations between Nisargadatta Maharaj and people who visited him while he was alive.
I have read it and studied it. This sentence is the main I have with me all the time, from a book of over 600 pages(in Greek).
But, I am me and you are you, John, so, I feel that you might find something in it, when you will be in the mood of reading it.