The best advice I can possibly give you is to stop caring whether you miss some review items sometimes. This is my second time on WK; last time I was up to level 12, and I used to agonize over each answer and experience an actual jolt of painful anxiety whenever I’d get one wrong. That sucked a lot and eventually led me to quit. Now I try to just enjoy it when I’m right and shrug when I’m wrong.
It doesn’t matter if you get one wrong. In fact, it’s just an opportunity to get more exposure to the item. No one is monitoring your performance so you’ve got nothing to be embarrassed about when you miss something easy. I posted a screenshot on Facebook when I got a 250-item review 99% right, but if I’m 70% or whatever then no one has to know.
The only things that really matter to get right in terms of progression are the radicals and the kanji, not the vocabulary, so you can feel free to get vocab wrong as much as you want and it doesn’t matter. And whether you level up in 8 days or 12 days… who cares. It doesn’t matter. As someone else said, this is a marathon and not a sprint.
Be happy and proud that you’re making progress and just do what you can when you can. The devil’s trap in this is that you try and care so hard that you get overwhelmed and start letting reviews pile up because you’re afraid of how much you don’t remember and are afraid of making mistakes. But then you drop WK for a year (like I did) and you regret it. Just make the mistakes and learn from them and keep pressing forward. You’ll be amazed at the progress you make.
75% recognition of 2000 kanji is 100% recognition of 1500 kanji. How many kanji do your friends know? Your parents? You’re way ahead of the game if you just keep moving forward.