This is expected, normal, and precisely why WK introduced the extra study feature. We all need frequent review of newly introduced information. Trust me: sheer repetition eventually suffices if mnemonics fail.
I honestly think we often expect too much of ourselves. Attitude is important.
Expect that a small number of items will get 8 reviews and be burned forever. Most will require several more than that. And some will require dozens upon dozens of reviews before they finally stick. The job of an SRS is to let you space out the repetitions of the easy stuff to give you more reps with the hard stuff. Incorrect answers are the most important part of the process — try not to feel bad about them.
That said, too high a percentage of incorrect answers will grind you down. I rarely do more than 5 lessons a day when new kanji is being introduced (and up to 15 or 20 per day when it’s all vocabulary just hammering home the pronunciation of already learned kanji).
Believe it or not, it does get easier with daily practice. Somewhere around level 30-40 I noticed that I rarely even bother with mnemonics any more and just rely on sheer repetition (your brain starts to recognize patterns).
I’d strongly suggest using extra study before or between sessions. Or do what I do and use the GanbarOmeter to launch self-study for items in stages 1 and 2.
Mnemonics can help to get things into short term memory, but the goal is to instantly and effortlessly read things “in the wild”. Sheer brute repetition is the best way to accomplish that, in my opinion.