Would you enthusiastically re-hire that employee, given a choice?
Would you take that job again, given a choice?
When it comes down to it, that’s the real test of whether a decision was regrettable or not. If you look back and say “Given what I knew at the time, I’d have done the same thing,” then there shouldn’t be any regret. Those are your “lessons learnt”. What will you change next time? What information/strength/perspective were you missing that would have helped you make a better choice?
But if you look back and say “That was a mistake, and I knew it even back then,” - there’s your true regrettable moment.
I have a bunch of regrets as well as lessons learnt. I am resolved to make fewer regrettable mistakes and more learnable mistakes.