Go deep into your study of people, and you'll discover unsuc-cessful people suffer a mind-deadening thought disease. We call
this disease excusitis. Every failure has this disease in its advancedform. And most "average" persons have at least a mild case of it.You will discover that excusitis explains the differencebetween the person who is going places and the fellow who is fellow who barely holding his own------This is actually poorly written English and bad philosophy also :) -- I don't think starting a sentence with a verb "Go" is proper, nor starting with "Every" nor with "And" . Let me try : If you study people deeply, you may discover unsucessful people have a thought disease that is numbs their mind. We (whomever that is) call the disease "excuse-itis" . You will discover (this is the author telling you how you will think, like a hypnotist) ....
Anyhow you get the idea it MD probably mean someone who is "closed minded" or "numb" to new ideas.
PS: "barely holding your own" means is barely able to not go crazy , or barely able to survive (financially or otherwise)