Do you mean lessons, or do you mean reviews? It kind of sounds like you’re talking about reviews.
Regardless, for lessons, I used to do smaller batches over time. If you want to go fast at wanikani, the lessons could be in significantly higher batches. I used to do half of my vocabulary in one sitting every time I had lessons because I was going at near top speed. If you want to go slowly, just do a set number of lessons every day. It’s easier to do 15 than 45, right? It’ll be slower but it won’t overwhelm you as quickly.
If you were talking about reviews, slowing down on your lessons will help. If you are doing less lessons than less reviews will show up all at the same time. Going back to what I said about going fast, I have the unfortunate issue (that could have easily been prevented) that I get a lot of reviews at one time. This morning, I woke up with 275 reviews to do. It’s not ideal but it works for me. You need to find your own groove and figure out what works for you.
As for motivation, do wanikani everyday at the same time. Set aside some time specifically for it every day. It’s easier to do something when it’s happening at the same time everyday, right? Just like going to work or school. You don’t want to do it, but the more you do it at the same time everyday, the more used to it you get. If you need motivation for reviews, do it at set times as well. A lot of people on here do their reviews 2-3 times everyday. Once in the morning, once at around lunch, and once in the evening. That tends to be around what I do.
Anyway, good luck! I know it can be hard to get through wanikani, but if you persevere it’ll make you a lot better at your Japanese. And like I said, find your groove. Figure out what works for you.