Even if he was not his mate our baby dragon deserves to love again and be in peace. That’s probably what past taehyuk would’ve wanted for him I think
I mean even at the begining of the story, hyun oh was already losing hope in finding the reincarnation of his lost lover, so I think that slow progression of letting taehyuk in was allowing himself to fall in love with someone else (not meaning he forgot about his past lover) isn't necessarily a bad things, even if taehyuk didn't turn out to actually be his lost lover.
I can still see your frustration don't get me wrong tho!
Actually he could fall in love all over again because it was Taehyuk. I don't think if the "second person" wasn't Taehyuk he wouldn't have fallen yk. Reincarnated Taehyuk still has the same personality as the "master"
It kinda ticks me off to think that Hyo-un loved “Taehyuk” unconditionally in his past life yet he still manages to fall in love with “another guy” in this life. What if MC wasn’t his “sir”? Wouldn’t it mean he betrayed his own feelings? Luckily it’s a romance, so Taehyuk is his mate.