I second this. I've been studying for a year and I've had times where I had to take breaks ranging from a week, up to two months. What I mean by taking a break is that I'm just reviewing my Anki cards, but not learning any new cards. That way even though I'm not moving forward, I'm at least not taking any steps back either.

I try to think of willpower as a resource that can be depleted. Sometimes I have to take breaks, so I can let my willpower recharge.

There's also a willpower cost for everything and some things cost more willpower than others. Learning new things cost more willpower than reviewing old things. Boring stuff costs more willpower than fun stuff.

I use to use sentences from the book 2001 Kanji Odyssey as a learning source, but I eventually burned out, since most of the sentences were about boring business related subjects. Now I have separate Anki decks for TV shows, manga, and Satori Reader. Since they're separate decks, I can choose what kind of material I want to focus on each day. These are more fun and motivating, since I have the goal of understanding an entire episode, chapter, or article.

Another thing that helps a lot is building habits. I have a habit of doing all my Anki cards in the morning while drinking my coffee. I created more Anki cards whenever I feel like it, but I always make sure I at least have enough to study for the next day.