When it comes to happiness, there's no such thing as "true" outside of your own cognitive perceptions. I mean, what would that even look like? Your own happiness is just an emotion happening inside your head; how could any other person, without access to your head, tell you that you're "wrong" in perceiving that?

Now, it's possible for someone's actions to suggest that they have some subconscious motivations that are in conflict with what they say makes them happy. And it's also possible for them to grow in ways that overcome these cognitive distortions, such that chasing an ambition over all else is no longer fulfilling for them. But even if that happens, in the moments before growth they still were not losing out on anything, because their psyche at the time didn't care about what they missed out on.