I got through Wanikani by doing all my reviews everyday, no matter how tired I was and by doing a limited amount of lessons everyday (fx 10), so that the flow of reviews was always even (if you do 50 lessons at once, you’ll be kicked in the nuts by them all at once again, which can be exhausting). And if you KNOW you’ll be unable to do your reviews for awhile, use the vacation mode!
I don’t necessarily agree with the others about slowing down, it’s more about steadying your effort. It’s also good to have a goal for how quickly you want to finish Wanikani, if you make too slow progress that can be discouraging as well.
Wanikani makes a great effort at making learning fun, but in the end it’s sheer hard work and dedication. You shouldn’t look for motivation but rather habit. Do reviews in the morning with your breakfast and then again at a particular time in the evening, or just do a small batch at a time throughout the day. It should be as natural to you as brushing your teeth in the morning - you do that even if you’re tired.