Disappointment?

If there's one thing I've discovered over and over again that characterizes many people I know - some of them extremely well - it is that they are disappointed. They are living their life in a perpetual state of disappointment, with no end in sight.
It's mainly one of two things. Disappointment with the way their life pattern or career has turned out (or not turned out, as the case may be), or disappointment with themselves - perhaps the way they are, the way they look, or possibly with what they have done or failed to do.
If we were all honest we could probably point to at least one of these and identify closely with them, but how do we avoid dragging ourselves into a life in which the sense of "if only" is a dominating factor?
In the Bible, Jesus says that he has come that we may enjoy life to it's fullest extent (John 10 verse 10) and we have to somehow acknowledge that we will never live up to our own exacting expectations, let alone God's perfection, whilst still maintaining a positive outlook on life.
I may not know much, but I do know that disappointment sucks the life out of you until you are just a hollow shell, constantly looking back to the past and wondering what might have happened if....
Is that really the way you want to live?