I'm having to have a really good think (difficult I know, bear with me )
The only thing that honestly springs to mind right now is being able to avoid the pain that I caused my first Husband (and myself) when I decided to leave him but that would have meant that we didn't love each other which simply wasn't true.
I'll be back later if I can think of anything else.
Oh, maybe learning to drive a bit sooner than I did (I was 27 when I passed my test) I think I would have enjoyed the extra freedom and independence that gave me, but I suppose what I never had at the time, I never missed (if that makes sense?)