Excellent slang example. タラレバ話 ('tarareba talk') is a Japanese-specific term for 'hypothetical regret talk' — combining the two conditional endings ~たら and ~れば, which are the grammar forms used to construct 'what-if' statements. A noun derived from grammar forms, similar to English 'coulda-shoulda-woulda.' 留年 ('repeating a year') is a specific university/school term. んの is the ultra-casual contraction of のだ.