Consult not your fears but your hopes and your dreams. Think not about your frustrations, but about your unfulfilled potential. Concern yourself not with what you tried and failed in, but with what it is still possible for you to do.Attr. to Pope St. John XXIII
A Google search turned up the usual suspects, none with citations (at least in the first few pages of results). Wikiquote has it on the “Unsourced” list on the discussion page for Pope St. John XXIII. It mostly doesn’t appear in books before about 2000. The oldest citation I can find is in The Ligourian for 1972, which is only available in snippet view, so I can’t see the full context.
Since no one at Wikiquote could find a real source, and since I couldn’t either, and since this sounds more like pop psych than anything else, I’m going to call it doubtful. If anyone has access to The Ligourian, I’d love to know what it says.