Yeah, I totally understand this. I, like you, feel such erratic changes in mood that I never know what I’m gonna get the next day I wake up. Sometimes I awake with dread looming over my waking soul, and the next day I am grinning like an idiot.
Just remember, the bad days don’t last. They never do. Give yourself a week and do (or don’t do) whatever you want (or don’t want). After the week is over, evaluate how you felt about it. Guilty? Shitty that you gave in to the sadness? By the end of your week, you might even find some motivation creeping back.
The trick is sometimes not to fight it. The more you do, the less motivated you will get. It’s a push and pull game.