Wow, so Doughnut turns out to be an MV picturing a tragic relationship?
Yes, the way I read it.
I think it's a song about a relationship that they thought would last forever (an endless love loop), but in the end it wasn't meant to be.
A lot of the lyrics talk about feeling lonely and missing the other person and wanting to be with them (hold in my heart in the shape of you), but they aren't actually there.
So the doughnut in the end is showing they found out that the relationship is ruined. And that's why Mina is sad.