In certain sciences like astrophysics it's often impossible to measure things directly. You can't really fly out with a giant galactic tape measure to figure out the size of the Milky Way, instead you measure stars that pulsate in proportion to their energy and use that energy and how bright they appear to figure out how far the light must have travelled. Similarly, you can't measure the mass of a black hole, you rely on the effect its mass has on nearby bodies. In fact, you basically can't detect black holes at all except by their effects on things around them.
It occurs to me that sometimes people have the same property. You can't really measure someone's intentions, personality or abilities. Instead, you have to measure the indirect effects those qualities have on the world around them. There's no perceivable difference between an amazingly intelligent person who never says anything and an amazingly dumb person who never says anything. Someone who would do amazing things if they ever had the chance but never gets that chance has still achieved exactly as much as someone who never tried at all.
A possible (depressing) conclusion to that line of thinking is that you don't exist except as measured by the opinions of others and the results you achieve. But those measurements are so unreliable. In poker they talk about a bad beat: a hand where you do everything right but lose anyway, or vice versa. How can you possibly be happy with your decisions if all you care about is the win? And how can you even have an identity if it's just what people think you are? There is, however, one (and only one) person your internal state does make a difference to: yourself.
I think to be able to function effectively in the face of adversity or indifference you have to be able to measure yourself on your own terms. If you can define without reference to the outside world the shape of good and bad, of success and failure, then you're also free from trying to control those external factors which are, fundamentally, beyond your control.
Measuring yourself by your effects on the things around you may be simple, even easy, but the end result is being dependent on those things for your happiness, and even your identity itself.