Imo, the MC is not crazy. Because of society's pressure and expectations the ML was unable to be with MC and reveal his true sexuality (chinese society is harsh). Which is why I think the ML is actually a doppelganger. I have heard that doppelgangers appear when someone has a very strong unconscious desire or fixation which the main self is unable to realize/vent. Could also be a "living ghost" (ikiryō (生霊)) which is pretty much the same idea. I think the ML might have loved the MC, possibly since before he knew the MC's feelings. If it really was like that then finding out the person he liked but couldn't be with, liked him too would've had a huge effect on him.

It's even possible he was pressured into marriage by his parents or that he was even suppressing his true sexuality subconsciously. He doesn't have the courage to give everything up for love or to embrace his true self, so deep inside he's very frustrated. So he harshly refused him (his homophobic rejection might be cuz he is in denial or afraid to face his feelings properly) but in reality he kept thinking of MC. Then he probably found out that the MC began sleeping around and also noticed the MC became indifferent to him and no longer seemed to like him, which was a huge blow to him specially since he was unable to do anything about it.

Which is why his obsession and frustration created the doppelganger to find a release, but his main self is unaware of its existence. I think that's why it says "I love you, but I can’t be with you. If I can split myself in two, I will be with you" it refers to the ML's wish. Not the MC because he really did moved on with his life.

This oneshot honestly can be interpreted in many ways. It's either MC has a disorder (it's tragic and really sad now that I think about it) OR the ML really split himself (it sounds so horrifying, I'm actually thinking about Ito Junji lol) and has both the MC and the girlfriend. At first, I thought that the disorder thing was likely, sadly, considering the setting but when I looked at the summary I had doubts (It's either that MC had just imagined the ML saying those words on the summary or it's really genuinely the ML saying these things)

But now, unfortunately, I'm more leaning on the thought that MC really has a disorder when I observed (basically re read) the MLs actions... it seemed so unnatural and unreasonable, it's just so likely that the MC is making up the ML's reasons, as to why ML rejected MC in the drunk confession scene, in his mind...

It's all just a fantasy and MC gradually integrated it into reality thinking that it was real and true but REALLY those moments of ML being kind to him after the drunk confession was all just not real.

or PLOT TWIST, ML is a whole different person entirely! Maybe he's a ghost that fell in love with our MC and decided to look like the guy MC has a crush on (It's cute but I'm also getting chills.. suddenly thinking about Ito Junji's works (ಥ﹏ಥ) or maybe ML is a highlevel system transmigrator that used his system to get two bodies, so that ML can complete his mission (regarding the gf) while also simultaneously flirting with MC.