I'm not sure if i got it correctly... correct me if i'm wrong .... So our main confessed ...

And it could be that his crush and that manager have the same name, which is why our main slept with him in the first place?

Here is how I understand:
1- > The first page of chapter 2 where his former crush part ways with him seems to happen when they are already Adults (see clothing in high school they wore gakuran) so to me he only confessed to his friend many years later and for some reason his friend couldnt continue their friendship.
2-> when he was in a abusive relationship with Ishijima (no love, only lust he used to call out his friend name during sex)he was still friends with his crush and his crush still didnt know about his feelings. Also the MC was in another city far away from his friend.
3-> We still dont know exactly how abusive his relationship with Ishijima was nor why Ishijima left him nor why his childhood friend decided to end to their friendship. Also yeah Ishijima is back now.
PS: to me when his friend went out with a giril in highschool he had no clue the MC liked him yet. Also his friend name is Hisashi and the jerk name is Ishijima.
I get the feeling Ishijima was responsable for ruining their friendship like directly responsable but this is only a supposition.

Actually yeah they both are hisashi

I see...

Thanks for your reply!