I think Jiho is talking about the feeling of loneliness and isolation. The fact that he actually mever felt that sense of equality at the orchestra but pretended to do so in order to seem "normal" and like he connected to those around him just so Young wouldn't worry about him, it's that feeling that he's acting to blend in that makes him feel lonely. So I think the "biggest difference" is Jiho talking about the loneliness and emptiness that he can't get rid of no matter how hard he tries. I think it's that same feeling of when u know how ur supposed to think and that in reality ppl arent judging u or the crisis isnt actually as big as it seems and knowing twchnically how u should approach something, but not being physically capable of forcing ur mind to actually do that. That was my experience before i found meds that helped me actually put the tools from therapy into practice, and i think that's what jiho's going thru here. Just not feeling how he thinks hes supposed to and then spiraling because he hates that he's like this and it makes him feel even more alone. Basically "ive got major depression and u dont" is the oversimplified difference (from my understanding)

I’m lowkey having a hard time understanding when young says that’s the difference between you and me in ch 18. I’m thinking it means, jiho thinks everyone has it all figured out but young can tell it’s just a facade. idkkkkkk i’m just trying to make sense of it.