exactly, there’s not a single part of
me that believes heejae is anything other than miserable. right before their split, he admits to himself that he could survive without garam but wouldn’t be happy. he wasn’t happy before garam and there’s no reason to believe he would be after. i can’t imagine he would search for that sound or feeling from anyone else when he already knows it’s out there. he’s either too prideful or too hurt (or both) to get it back.
yuuuuppp!! the sentiment that he wasn't miserable the past 4 years would be completely against anything we learned about him so far.
I am constantly bouncing back between the possible reasons why he didn't try to get Garam back. Either pride, hurt feelings, self-hatred because he thinks Garam is better off without him or him just thinking the prophecy he kept repeating to himself since he was a child (I am a knife, I only need music) is now once again fulfilling itself, so there is no redemption for him. He feels reassured in what he kept on telling himself to deprive himself of human connection and love but also to deal with the fact that he was deprived of love in the past.
Next chapter will be punch in the gut again, I just know it.

(posted this on twitter but kinda wanted to give my two cents on here, too)
idk maybe i am too Heejae pilled but i really don't share the thoughts or fears that he just carried on like nothing happened, found another student or just fucks around.
i think he was/is miserable as well and just retreated into loneliness in his way, barely talks anymore unless obligated for work events etc.
I will be even more devastated next chapter, but I think Garam leaving had an immense impact on how he carries himself.
he also probably smokes even more than before. we rarely saw him smoke around Garam when they were together. he smoked at events or when he was waiting (for Garam) or to get a small break at events. now we see smoke in 2/3 WIP panels BDK posted of him (on twitter).
he tries to keep up appearances of carrying on like before bc he has to and probably because of his pride. but it just doesn't make sense to me character-wise that after Garam left him and all of his trauma about the manager who's ghost HAUNTED him 10 years later and made him go insane, that he just keeps on being his cocky self and goes on to search yet another Gem. i think he abandoned his mission and probably thinks that Garam leaving him is what's best for Garam, maybe he even explains it with his pride and ego that he wouldn't chase him, but I don't think he had a single happy day after Garam left him and he keeps on doing his job, composing music in private and chainsmokes cigarettes.
i mean he loves Garam DOWWNN he just coudn't say it for reasons I already wrote half an essay on here (twisted understanding of being incapable of love, negating his humanity by depriving himself of feeling human and therefore openly accepting that music isn't the ONLY thing he is ALLOWED to love as a knife) and he genuinely shared a deep connection and companionship with him. it literally doesn't make sense that he doesn't give a fuck about him. he wasn't able to tell the person he loves most that he loves him and now he isn't able to tell the same person (Garam obv.) that he NEEDS him in his life and wants him back.
Heejae thinks he is a KNIFE and not worthy of love and he deluded himself into thinking that music is all he need and music will always be there for him. but that's obv. not true and he keeps punishing himself and repeating his patterns of pushing everything human away, esp. his own humanity, so he doesn't have to face the fact that he deprived himself of love all his life and that it wasn't necessary. it's hard to accept that your biggest enemy is yourself.