This sounds very much like The Sun Grows Cold by Howard Berk, published by Dell in 1972.
It opens in the Medical Division of a mostly-underground government installation called "Complex One," where large numbers of people who have gone insane for some undisclosed reason basically have their brains wiped and new personalities developed. There seems to have been some sort of war as well, but no one talks about it.
The protagonist, a "reprocessed" man given the name of Parnell, still retains enough of his old memories to try to illicitly access the Archives for all of Complex One in order to find out the reason behind its existence and the truth about the war, and escapes after being captured while trying to enter the access code.
After a long post-apocalyptic adventure, Parnell finally remembers the full access code to the Archives. He is allowed back to Complex One to learn the truth, which is that the mass insanity (called the "Compression Factor") was a genetic response to human overpopulation -- and that, in his previous life, he had been the psychologist who had discovered the Compression Factor and replicated it in rats. He had theorized that a deliberately-engineered nuclear holocaust to reduce the population could cure the resulting remnants of mankind of the insanity, and seeing his hypothetical suggestion brought to life in the form of mass destruction finally drives him around the bend.
The novel ends with him back in the Medical Division after a second treatment, content and child-like, remembering nothing of what happened.