On closer reflection; on second thought — used when reconsidering or recalling something and often changing one’s view.
よく考えたら、今日は予定があったことを忘れていた。
On second thought, I forgot I had plans today.
よく考えたら、その案は実現が難しいかもしれない。
On closer reflection, that plan might be difficult to implement.
よく考えたら、彼の言っていたことは正しかった。
Come to think of it, what he said was correct.