Yuta isnt bipolar, he tried to "hide" himself, he wanted a revenge, he wanted to be "sweet", cunning, to fool Seunghyun but he actually wasn't that strong, Seunghyun could still read him & now after Seunghyun show what he understood about Yuta, Yuta panicked.
the scene where he had hard time breathing is panic attack. He show this as a failure, he still refuses to admit that Seunghyun bei genuine even though deep down he was & still is have feeling for Seunghyun.
the reason why his behavior or words kinda "changing" is bc that's part of his attempt to be "cunning", part of his plan. But, again, his feeling is still showing itself & Seunghyun's feeling also has begin to catching up.

is the grey head bipolar or something cause im a little confused on the dynamic of this relationship