I think that he was thinking about his bad influence on Niina. It's not like he don't love him now... probably. (▰˘◡˘▰)
It would be one of this sicko story with plot twist, but this sensei isn't Harada-sensei, so it's fine... ┑(￣Д ￣)┍
For me, it seemed like he was reminded of the old time, not that he is only in love with the younger version and dislike the older version. I mean if he is, he wouldnt stay with him and find another high schooler.
I disagree. It's not that he doesn't love the uke, but because he does his past cruelty hurts him more.
The ending comtains a feeling of emptiness and melancholy that I think fits perfectly, because he's realised his feelings and he understands just how his past depravity has twisted his lover. He has to deal with the fact that he was the one who shaped him into a colder and wounded adult. I like how the past isn't just glossed over and instead it makes the seme have to confront his own actions, and acknowledge how he failed the uke.
I did not like the fact that sensei is thinking of the younger version of Niina like he does not like the older version.
Niina was bad ass even before as he manipulated sensei in highschool.
If you love someone. U love them.