I agree with what everyone above has said - don’t do lessons now, focus on doing reviews. Never ever ever do any lessons when you have pending reviews. That will just make your situation worse.
Keep an eye on your apprentice count and find an upper limit and stick to it. If your number of apprentice items goes over that limit, then don’t do any lessons. Keep doing reviews until you get that number down. Your daily number of reviews is roughly your number of apprentice items plus 10% of your guru items. If you feel that you don’t want to do much more than 100 reviews per day then don’t let your number of apprentice items go over 100.
One more thing, take a look at wkstats.com and see what levels you struggle with. You say that you have been stuck at level 12-17 for a long time. Maybe you should consider resetting back to level 12 or 13 or 14 just to get back some control.
How many lessons do you do every day? For me it helps to set a number and then do that number of lessons every day, no more. My number is 10 lessons, and I never let myself do more than that per day. And if I have pending reviews I don’t do lessons.
Try to do reviews in small batches several times throughout the day. 20 reviews 5 times per day feels easier than doing 100 at once. And be patient - it will take some time to get rid of your 400 reviews. Many of them will come back to apprentice and your apprentice counts will increase. Just breathe and work through it.
For what it’s worth, I know what it feels like. I was in the same boat a couple of years ago. Drowning under reviews, doing lessons only to feel like I was drowning even more, again and again and again. In the end I just gave up and left Wanikani. When I got back I reset back to the beginning. Now I’m slowly climbing back again, never doing more than 10 lessons per day. So far so good.