But the reason why I would've stayed is simply if my partner supports my decisions/promises/goals/wishes.
This was my perspective on this too. I'm not sure.... maybe it's because I'm viewing this more from the perspective of an interpersonal relationship, instead of what is potentially best for the inquisition (because what is best for inquisition is arguable).
Yeah, he's already good as he is... I really feel that he blames himself a good deal for Haven though, even if he doesn't say it that way.
The thing is, as he's having high & low moments, there'll be moments when he'll be in a vulnerable state and/or unable to perform correctly his duty & he knows this very well. This might not have much impact on a normal day in SKyhold but imagine him having "bad day" during the battle at Adamant? Or Arbor Wilds? The risk he's taking, it might cost not only his live but also Thedas' fate. I guess that most of his anguish in Perseverance comes from this realization.
He has an outburst about how he must do absolutely better at protecting Skyhold, which hints to his state of mind.
I got his scene after the Adamant, which just added to the whole concept of him panicking. Sometimes the timing of the quest works really well.
On the other hand, you know, I get it, much depends on him but he is not alone in this, there are supportive people around him, even if you don't romance him. Sometimes I think that he is too used to be alone and rely only on himself (what happened in circle, in kirkwall etc.)
edit: I just run that scene with different options, there is really good range of solutions (and how he response) even when telling him to continue without lyrium, you can be kind and you can even be "don't be a chicken!" about it.