Regret

(ML was quite rough with MC in his first life)
ML is force wed to MC because the empress doesn't want him on the throne. He spends his first life ignoring MC until he was blamed for a crime he didn't commit. He sees that MC stayed until the end and even sacrificed his life for ML. They return to the past and ML is set on loving MC this time. MC does not remember his past life.

ML rejects MC because he's too afraid to love. They make up at the end.

MC goes into a coma for 5 years and goes missing. EXML, instead of searching for MC, immediately abandons ship and moves on from him because he's convinced that MC left him for some reason. MC shows up 5 years later and ML treats him horribly without even asking why he was missing. MC starts dating another guy who is great.

ML leaves MC for a while because he wanted a ''normal straight life''. Of course they go back to each other and it's cute.