I feel exactly the same. I'm not bipolar, I'm CPTSD, but sometimes I'm literally a different person by the end of the week, and I'm extremely aware of its causes.

Disliking my job, as others have said, yes.

But it's also that the negativity against myself wears off a little with every day and hour that passes (possibly because I get to see myself being OK at my job, and realise that nothing too terrible has happened).