On the zeroth thing: Recall that there’s broken and broken. The degree of sympathy and the extension of a hand tends to be qualified by how much there’s actual dysfunction and how much you’re willing to try anyway.
Someone with real, quantifiable problems who tries to get shit done despite them gets a lot more of those than someone who appears to be making a career of whining about how X did them wrong and someone else should fix their shit, all too often with a side order of digging themselves deeper.
On the first: A subjective measurement can be wrong in exactly the same way that an objective measurement can be wrong; if you’re using a broken yardstick, it ain’t measuring yards. Likewise, the accuracy of your sense of your own (subjective) awesome depends on your brain having a functional introspective awesomestick, because otherwise you aren’t measuring what you think you’re measuring.
Even subjective metrics are only meaningful if they are what you think they are.
On the second: …
…okay, let me flip this out to a larger context for a moment.
Much as I generally dislike trying to interpret other people’s thoughts for 'em, it has been occurring to me that many of your queries on this sort of thing could be answered by addressing one underlying confusion, if it happened to be the case.
Self-determination and obligation aren’t the end of morality.
They’re the beginning.
They’re the crunchy ethical, deontic center at the middle of your delicious morality truffle, which come wrapped in layers of negentropy (via such things as the Five Noble Precepts) and aretaic injunctions, primarily the Nine Excellences: Unity (encompassing integrity and authenticity), Honor (encompassing self-consistent integrity, justice, truth, and clemency), Duty (as very distinct from obedience, and encompassing liberality, generosity, loyalty, and tenacity), Reason (encompassing self-honesty, wisdom, and craft), Courage (encompassing responsibility), Harmony (encompassing beauty, courtesy, kindness, refinement, and the appreciation of excellence), Right Action (encompassing ambition), Liberty, and Dignity (encompassing pride, propriety, and temperance). (Also see relevant section here for additional nuance in some areas.)
And it is to these aretaic outer layers that one is best off looking for motivation, since they concern themselves primarily with SHOULDs, whereas the ethical core concerns itself primarily with MUSTs and MUST NOTs.
Now, to bring it back to this particular example: to watch someone depressed - even a stranger with whom one has no other connection - abuse themselves via incorrectly punitive self-valuation would self-evidently be pro-entropic, unjust, untruthful, ungenerous, and unkind.
Ethically, you need not do anything about it, and are obviously forbidden from doing non-consensual things about it; as a daryteir, a gentlesoph, you most certainly should do what you can.
