I think you’ll recover just fine :3 The advice in the replies above should get you there in no time. Also try to do some leech training/extra practice on the side to freshen up your memory of some of the forgotten material.
I also got stuck on level 21 for a few months (not quite a year) - I can tell you it’s not at all that bad :3 Personally I didn’t want to drop down levels, so I just slowly worked my way through the review pile - bit by bit, just as much as I felt like doing every day. Once that was cleared, I just moved onto lessons - and started clearing that pile slowly, too. Takes a bit of extra time and patience - knowing that you won’t necessarily level up soon can be discouraging. But you can do this! :3 Just make sure to give yourself time to get back on track,
Oh, and let me not forget: don’t know if you’ve seen this, but there is the study buddy race thread here - come join us for some good vibes and a supportive atmosphere :33 We’re all aiming for level 60~