How do I slow down?

It also helps to do the same number of lessons every day. When you do that, your workload will level out and become predictable. Doing 20 lessons a day and it’s overwhelming? Try 15 lessons a day instead. And so on.

I generally recommend using [Userscript] WaniKani Lesson Filter as well, so that you can mix kanji and vocab lessons every day. I personally did 3 kanji and 9 vocab lessons most days for my second half of WaniKani, which had me level up about every 14-15 days.