You’ve had a lot of good responses in this thread already, but I’ll just chime in with my two cents.
There are a lot of high level WK users here in the forums, and a lot of users who are using the ‘gamification’ of WK to race through to Level 60 as quickly as they can. Don’t feel like you have to compare your progress to theirs! If the race to the end doesn’t work for you (it doesn’t for me), don’t do it! Don’t worry how fast you’re getting there, just that you’re getting there.
On that note, too, progress isn’t linear. Some days you will feel like you’ve absolutely nailed this Japanese beast! Some days the beast will eat you alive. I’ve been there. It sucks when you have those frustrating days, but it’s all part of the process and it’s important to keep a positive attitude and get back on the saddle. The beauty of the SRS system is that getting something wrong on a review isn’t a failure. It just means that you haven’t quite learned it yet, so the system will make sure you see it again a few more times until it sticks. It’s not a failure - it’s just part of the process.
Set yourself goals. When you have those frustrating days, look back on what you’ve achieved already. You know hiragana and katakana! Remember when that seemed impossible? In a few months’ time you’ll be burning kanji you didn’t think you’d ever be able to burn.
Don’t rush to complete all of your lessons at once. Some of the most common advice you’ll get here is to find your sweet spot. For me, I limit lessons to 15-20 a day. Any more than that and I find that I can’t remember them. If you’re getting too many reviews and it’s starting to overwhelm you, don’t do any more lessons until you’ve guru’d a few items and got it back under control!
Wanikani tends to recommend starting grammar around level 10, but I’d recommend starting straight away. I use the Genki textbook. By understanding the grammar as well as learning kanji and vocabulary, you’ll be able to put everything you’re learning on WK into context, which will help your WK learning in turn!
So, uh, that was a lot more than two cents, but I hope it’s helped. Just know that you’re not alone in feeling this way, and your feelings are valid. Japanese is the hardest language to learn! But there’s a lot of support here. You’ll get through it!