Sure/ for sure

Prolonged unemployment can create serious psychological problems that, in the long run, actually contribute to continued jobless. In a society that stresses the relationship between productive work and personal value, it is easy enough to equate long-term unemployment with personal worthlessness. That is, in fact, precisely what many unemployed men and women begin to do. Out of a job for a year or more, they begin to see themselves as worthless human beings without any value to society. In what amounts to a vicious cycle, their sense of personal worthlessness further diminishes their chances of gaining employment. Sometimes they stop looking for work altogether, sure in their despair that no one will hire them.
[Source: Reading for Results Ninth Edition by Laraine Flemming]
I'd like to know the difference "sure" and "for sure" when "sure" used as an adverb.
Thank you in advance for your help.