This reply is sort of off topic, so apologies
When I am depressed I have that same "What it comes down to is that I don’t seem to have a purpose." feeling. That feeling of being here, but not being a part of being here

I force myself to have purposes. Who would fill my bird feeders I if was not here. The birds count on my feeders, any they might die.

Small attainable things, but nonetheless a valid purpose

Does this make sense?