Time to make stuff up!!!
Feeling young again after TMP, he starts acting that way and successfully completes a second 5 year mission, and after a mandatory year off, signs up for a 3rd, but soon thereafter, the ship is nearly destroyed and he is severely injured in hand to hand combat that he is convinced his younger self would have won.
Despite this, he, the crew and the ship miraculously survive yet again - thanks this time to the heroics of Spock instead of the severely injured captain who wasn't even conscious to save the day.
Facing the prospect of nearly losing their most celebrated commander in recent memory, he is forced by the top brass at Starfleet to either accept desk duty or retirement. They aren't willing to ship him or the Enterprise to the far reaches of the Federation any longer. Kirk is convinced that by his failure that he simply is no longer the man that he used to be and is afraid - not that he'll get himself killed, but that he'll get his crew and ship killed because he just doesn't have it anymore. He won't admit it to anyone else. He must project confidence. Retirement would be giving up the only life he's ever known, and admitting defeat, so he accepts desk duty.
When he takes the ship out for the first time since re-assignment, for a training cruise in TWOK, his carefully constructed demeanor begins to crack with snide comments about gallivanting around the universe being a game for the young... catching all his long-time friends completely off-guard. The prospect of being on the ship again, even on a training cruise brings back bad memories.
There. There's the made up Chimp version, on the spot! Worth nothing, but fun for me to write.