When you learn the lesson it will be too late. 

By that I mean that by the time you learn the thing you wish you knew now, you'll either have already solved the problem that currently chains you, or you will have already solved it, 

The latter scenario might seem like a 

happy path

. You've grown! But there's also the shadow possibility of the latter in which you solve the issue but you still don't feel any resolve. Think of the kid who finally figures out how to get to the cookie jar only to realize that eating all the cookies in the world isn't going to make him any happier. More elated for the hour they binge on the cookies? Yes. But although people want to live longer and longer, life is still too long for most people. 

As in most of us most of the times index too much on the things that make us happy in a run that's too short compared to how long we will probably live. Probably not cookies by this age, but adult cookies. You know them, I don't need to list them here. 

By the time you learn that lesson it'll be too late.