Today is the only one that matters

Yesterday is irrelevant - it's already gone.  Learn what you can from it, enjoy the memories of the good bits, and glaze over the dull bits, and move on.  Tomorrow is irrelevant - it isn't here.  You can't worry about it, you don't know what it will do anyway, so what's the point?

Today, now today is a much different animal.  It is here, right now.  Are you truly doing what you want?  If you died tonight would you be happy (in a dead kind of way) that you ran well on your last lap of the human race?  Or would you be full of regret for never having taken that chance, never risked it all, stood on the edge, made something special happen, broken the rules, captured a moment, or just plain and simply lived a little?

Go on - you might just enjoy it.  Never put off till tomorrow what you can do today, because if you do it today and you like it, then you can do it again tomorrow   :)