Britney Spears is perfectly fine dancing on her own ... even if it's in the middle of a bathroom during a "horrible" sushi date.

The singer shared a grim update on her love life Sunday, when she uploaded a video of herself on Instagram dancing solo in a bathroom. As she explained in the caption, the vid was taken during a break from a date at an undisclosed sushi restaurant ... where she had a less than ideal time.

Per Brit, the food was "yummy," but the guy was "absolutely horrible" -- he allegedly showed no effort.

She added ... "Dinner sushi date but I have my own girl party in the bathroom." Instagram/@britneyspears

Still, Britney proved she was able to make her own fun, twerking and flashing her bum from beneath her black slip dress. She flipped her blonde locks from side to

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