A person who doesn't have the same main goal throughout life cannot stay the same person. But this isn't enough unless you know what that goal should be. Most people have confused ideas about what's good because they follow popular opinion without solid reasons. Their views can't agree with each other. Only goals that are limited by certain principles - like serving the common good - can work. Nothing should be called good unless it helps everyone. So the goal we set for ourselves must also serve the common good and help society. When someone directs all their private thoughts and plans toward this shared goal, all their actions will line up with each other. This is how they stay the same person.
He that hath not one and the self-same general end always as long as he liveth, cannot possibly be one and the self-same man always. But this will not suffice except thou add also what ought to be this general end. For as the general conceit and apprehension of all those things which upon no certain ground are by the greater part of men deemed good, cannot be uniform and agreeable, but that only which is limited and restrained by some certain proprieties and conditions, as of community: that nothing be conceived good, which is not commonly and publicly good: so must the end also that we propose unto ourselves, be common and sociable. For he that doth direct all his own private motions and purposes to that end, all his actions will be agreeable and uniform; and by that means will be still the same man.