Predestination: Meant To Be?

Sometimes I wonder at the outcome of our lives. You see, I'm a Christian, and so I believe in what the Bible says about my life:

... All the days ordained for me were written in Your book before one of them came to be.

-Psalm 139:16

What does that imply? That everything unfolding before me in my life had already been ordained for me, long even before I was even in existence. I guess that means that God already knew the decisions I would make in life, long before they were even placed before me, long before I even knew what they would be about.

So I wonder, is it worth going through all the motions then? If it's a matter of just a predetermined plot unfolding, what difference would it make that I lived through outcomes decided long ago? Or has God prepared a few alternate endings, where which one of them that I'd end up with depends on my decisions?

I cannot make sense of my life. I cannot reconcile why certain things have become as they have. I don't understand why some things of yesterday still haunt me in my dreams and thoughts, although circumstances have changed so much already.

Is this how it was meant to be? Or was it that I stumbled onto the wrong alternate ending, not so long ago?