Are there any words/phrases/idioms like "regret," but more specific? [closed]
There's l'esprit de l'escalier ("staircase wit"), which Wikipedia describes as "the predicament of thinking of the perfect reply too late".
After the fact
As an example
I only though of a good explanation after the fact.
After the fact generally means the information is too late to be of use. It's generally used to express a wish you did or said something differently, but it's too late now.