TITLE: Mathematicians who overcame academic failure to achieve success
QUESTION [17 upvotes]: Does anyone have any story of mathematicians who overcame "academic failure" or setbacks to achieve success later as a result of their perseverance? This is a soft question, that hopefully can inspire aspiring students.
New edit: I am especially interested in academic failure (e.g. failure of exams, failure in proof/ wrong proof, failure in getting academic jobs).  This is to narrow down the question so it is not too broad. There is another related question on blind/disabled mathematicians which is very good: Who are some blind or otherwise disabled mathematicians who have made important contributions to mathematics?
My ideal accepted answer is a relatively less well known answer (so that we all learn something new), supported by factual evidence (e.g.  a hyperlink to a page or a quote).
Some that I can list are:
1) Zhang Yitang, who worked in Subway (arguably a sort of a setback) but later proved a result related to the Twin Prime Conjecture.
2) Robion Kirby, who failed his oral Ph.D. qualifying examination (http://www-groups.dcs.st-and.ac.uk/history/Biographies/Kirby.html) but later proved the "torus trick".
Thanks! (Hope this question is on topic for Math Stackexchange..)

REPLY [3 votes]: John Nash struggled with significant mental health issues when he should have been in the twilight of his mathematical journey. He and his wife spent many difficult years battling with this illness. Slowly, Nash started to get back in touch with the mathematical community in Princeton; engaging with the students, his passion for mathematics never died. It was in his latter years that he was awarded the nobel prize in economics for his contribution to game theory.