Mistakes are a normal part of learning, especially in SRS but slow progress and a big workload are disheartening. If WK is for you (and it might not be), slow and steady wins the race.
Recently when I have >10 or 15 reviews, I use the clock wrapup to get a batch done:
[Level 4 and i just discovered the little clock "wrap-up" icon. AMA]
and then decide to go again or come back later.
I purchased lifetime membership and I guess I will continue towards level 60 but my immediate goals are about one level a month and to do that for the next 6 months. I might slow down further and if I need to, I hope to invest more time in writing notes with new lessons and also using kanji.sh printouts to write new Kanji.
I hope it works out for you!