I know you want to learn a lot of vocab quickly, but don’t touch the new lessons until you start clearing out your review pile. If nearly 600 feels like a lot to you, add new lessons to the pile and your review count will feel so overwhelming until you decide you have no choice but to reset. But don’t do that because you can clear those reviews no problem, just believe in yourself!
Give yourself a manageable goal - say 5 in an hour or 20 in a day or something. Work through the reviews so you only answer the ones you know immediately. Refresh every time it takes you too long to answer. It might feel like you’re denying the inevitable, but if it’s been a while since the last time you did reviews, your memory might just jog after doing some of the other reviews.
Don’t feel discouraged about how many you don’t remember. Pat yourself on the back every time your number count is decreasing.
Once you’ve lifted your confidence and you feel like you can take on more, steadily increase the rate of reviews you want to clear per hour/day. Continue not to touch the new lessons until you start feeling like this is fun! (You’re looking forward to a break from other things so you can get through your reviews kind of fun.)
When you’re actually looking forward to reviews again, you can tackle the new lessons, but you should only work through the vocab of older levels first. If there are certain kanji items in your reviews that you’ve been skipping or getting wrong, these vocab items can help give context to those kanji items.
Once you feel like you’re definitely at a manageable review number, tackle some new kanji lessons too. Just remember that unless you’re burning items, the reviews you get done will eventually return to the pile. If they’re in the apprentice stage, they’ll come back faster than those in guru, enlightened, etc. Adjust how many new lessons you plan to do based on how many reviews you anticipate getting down the road so you don’t get overwhelmed again.
I came back after 8 months to around 1300 reviews. I followed the exact steps I listed here and got my reviews down to nearly 0 (save for a few level 10 kanji I absolutely had forgotten) in less than a week. The trick is not to force yourself to do too many reviews at a time but to do a little at a time until it becomes fun.
At least in my case anyway, it boosted my confidence to see how many items I still remembered (maybe not right away, and that’s fine!), so it turned into a game for me to see how many I could get done a day, and I did 400~600. Even if it doesn’t feel fun for you, you can still catch up if you just take it little by little. There’s no rush!