Ariana Grande -- Donut Licking Video Was Rude Awakening ... I'm Gonna Be Better

Ariana Grande is embarrassed, humiliated, apologetic ... and a little bit puffy.

The singer posted an apology late Thursday for the stunt she pulled in the donut shop.

In the video, Ariana's face was still puffy from oral surgery she had this week -- but she poured her heart out about the donut incident, saying ... "Not here to make any excuses. I'm just here to apologize."

Ariana added that seeing the video was a "rude awakening" ... and said she was taking responsibility, and apologized for letting her fans down.