It was Hare, the Seme. The Uke was hallucinating about how the Seme was obsessed with him. Turned out it was the other way around. Seme liked the Uke back then, but Uke rejected him. So when Seme moved on, Uke started regretting his decisions and that when his hallucinations started.
Now that you reminded me, it indeed kind of resemble the one I wrote Hahahahha
https://www.mangago.me/read-manga/psycho_young_dream/
If you cant find xd

What if... Tsubaki has gone insane coz of too much regrets and started imagining things. What if none of the happy ending actually happened, Reiji actually got married moved elsewhere and they never saw each other again. Then Tsubaki, because he realized he loves Reiji more than anything in the world, has gone insane, and hallucinated and is now thinking he and Reiji are together. But the truth is... Reiji has marries someone else. Then Tsubaki being insane, has started getting sick and likes sleeping more than being awake, because in his sleep, he's with Reiji and they're together. He could touch him and love him all he wants. He refuses to wake up from reality until he finally loses his health too. When Reiji heard news about Tsubaki, he comes and visit him. When Tsubaki heard the real Reiji's voice, that's when he finally realize that he never actually got him, as now, Reiji's accompanied by his cute little daughter. Tsubaki being so heart broken and weak due to illness, heart failure called "Heart Broken Syndrome" has let out his very last breath calling out his greatest love's name. And he whispered the line "I will love you forever and ever. " and imagine a scene where he and Reiji are so happy in each other's embrace.
Then as he thought he's died...
He woke up in an unfamiliar place where people address to him as "Beastial Prince" and there is a beautiful male slave bowing in front of him. And when the slave looked up to him, he knew he looks so much like Reiji. And in this new place, the humans are the slaves while beastial are the ones at the top of the pyramid.
If you finished reading my nonsensical comment, poor you! I'm sorry HAHAHHAAH