I think of it like this...
As we go through life, it's like we are walking down a path. As we walk, we get to places where the path branches off into different forks. These represent the times we have to make choices in our lives. Some of them are big choices - which car do I buy - but others are much smaller choices - do I take my hat off when I get on the bus, or do I leave it on? Nonetheless, whether we are aware of them or not, we face many choices, and these are like forks in the path.
At the forks, there are gates closed across the entrance to each of the possible paths we can take. They are closed because we haven't gone through them, but they are not locked. We have several gates we can open and go through. Once we make a decision, we open one gate and step through. Once we do this, all the other gates we could have chosen are now locked, because we can never go back and make the choice again.
I believe that at each branching of the path, all the gates that are there are unlocked and available for us to go through. This is because each gate represents a choice that we could conceivably make, and there is nothing stopping us from, in theory, choosing any one of them. But if our lives were predestined, each time we come to a fork, we would find all the gates are locked bar one, and that is the gate we must go through. We do not choose which gate, the choice has been made for us. That is the difference between predestination and free will.