It comes from a book in one of my favorite series -the Dresden Files by Jim Butcher- called Death Masks and is self-explanatory:
It isn’t good to hold on too hard to the past. You can’t
spend your whole life looking back. Not even when you can’t see what lies ahead.
All you can do is keep on keeping on and try to believe that tomorrow will be
what it should be – even if it isn’t what you expected.