You and Torn Mind have a lot in common!
I do like him, he's a thinker.
Given that the entire discussion is an interpretation of lyrics, there can be many walls of text fleshing out the line and how it applies to particular contexts.
The term optimism can trigger many connotations along with its dictionary definition. Oftentimes, optimism comes with delusion as certain situations do not come out favorably.
I think part of this discussion is simply finding the correct adjective, and I don't think optimism is quite the right word. It is not that the anorexic would think their bodily analogue to a half-filled glass, which is their weight, is half empty. It's that it is totally empty even though it is half-filled.
I'm a positive but not an optimistic guy. I despise delusion and being delusional; even as a kid, I wanted to play with "real things". But, it is highly unlikely I will be in a mental crisis of despair like the OP or anorexics, depressed people, or some other manifestation of miscalibrated thought . I could...given the right triggers, be an explosive powder keg.