So you're suggesting the possibility that God created us for a purpose that we will never live up to? In that case it's irrelevant what our purpose is and we should just define our own existence. Like an adult who wants to be a chef but his father wants him to be a doctor.
If god really is just a sadistic bully. Then the best thing to do would be to show no fear and carry on with they way you want to live your life. Like most bullies, he will hopefully sense your resolve and move on to a weaker target.
In any of these scenarios, making the best of your present situation is the best thing you can do. The purpose doesn't have meaning.