How do I know that I am choosing the right purpose?

You don’t and you never will. Or at best only approximately. For it is others that determine our purpose. The best we can do is to respond each day to what those around us are asking of us on that day. So my purpose is always changing. Yet it always remains: to live for the other.

You will know if you have no regrets on doing that activity.You must make yourself feel proud for doing that act.Example:
You helped someone hold the door.This is an act of kindness that we can feel proud of.