It's probably some curse that's dragging it down. Check the Skill Menu, especially Curses and Perversions, there are a few that drag willpower down until some condition is fulfilled.
Viewing post in How do I deal with Curse III?
The reason this started happening to me was because I was hovering around 40 corruption. So between the curse -5 and the corruption -40, there's max willpower - 45 per new cycle, and then I started hovering around 10 or 20 willpower at day start once I had 'married' Herman. The ring has an effect that tries to force Aura to have sex with him, which builds and builds the longer you don't, so every cycle that you haven't activated 'masturbation is not enough' meant I was losing 45 + 50 max willpower despite using blessed waters or partaking in lewd actions.
Long story short, don't marry Herman until you're ready to be having sex with him, and try not to tank your max willpower too much through curses, or you'll have to consistently rely on emerald teas to function.