Happy gives me the impression she's going through "daily downers after evening highs".
Often what we use as a coping mechanism can incur in anxiety bursts the next day.
There's no logic to this, it's just the chemical rubber band in effect.
Addict behaviour essentially.
She needs self-discipline to both break the cycle and break her mental bad habits such as putting herself down, which while one may think gives a level of flagellistic reprieve actually only propagates one's mood.
I recommend the Aho Girl OP song for jarring oneself out of a bad mood.