It's one of those rueful, wistful sentiments: If I knew then what I knew now ....
A sentiment of wishful regret. With any luck, if I knew then what I know now, I might have sidestepped a lot of tom-foolery or serious error.
This morning, the sentiment strikes me as pure horseshit.
If I knew then what I know now, all that would have done is to open up whole new vistas for other more colorfully appointed bits of tom-foolery or serious error.
All that tom-foolery and error ... wasn't that the very stuff that brought you to where you are now -- supposedly more experienced and wise -- and wishing you had known then what you know now?
Regret is such a poor companion, not least when you stop to consider that what was regrettable is so richly informative in a present where there is no time to regret.