So fun fact: there’s no difference between nonlethal and lethal damage in my system. The players and gms are ultimately given agency to decide how an injury resulting from damage manifests.
Someone slapped the shit out of you and then knifes you?
Yeah, you’re gonna fall prone and have disadvantage on future attacks until you get that looked at, right?
Or perhaps they nicked a bad spot, and now you’re losing 1hp per turn.
Why should a stab that happened at the beginning be different than at the end? Interpret the results of damaging attacks however you want, let it make sense to you, then use that to write the story.
TL;DR. Hit points are to determine the threshold for when a problem occurs due to all previous damage that makes sense, damage that wouldn’t kill you is treated the same as damage that would, but you get to make up the consequences for reaching that threshold. This works for both physical and mental damage!