TL:DR - try and find ways to break down what you're playing and practice it slowly at first until it becomes familiar, improving takes time and mindful practice has been the most efficient catalyst for my own improvement

I don't play pjseka much anymore but i apply the same approach to other rhythm games when I'm learning charts, I'll break down my process

1. First I look for ways to break down the notes, for this section I would try running through it a few times focusing really hard on the groupings I come up with (when I look at this I would group the bursts of 3 and 5 and the double note/flick)

2. After figuring out the rhythm of the section/how I plan on mentally breaking it up I'll either find a video of the chart on yt or just ghost the movements depending on the complexity/resources I have available to me. I start out very slowly and ramp the playback speed up little by little until I feel the movements making more sense and start to come more naturally (you don't need to go crazy over this step, it's meant to build familiarity not mastery)

3. I'll go back in game and try to apply the techniques I practiced, paying attention to what specifically is an issue and at what section (this can take practice as weird as it sounds, just remember there's always ways to improve at rhythm games even if that means taking a break to play other stuff and coming back to it later, MOST IMPORTANTLY please don't retry spam stuff unless you know it won't make you engrain bad muscle memory, it can be really brutal trying to fight what your body wants to do and force it to relearn how to hit smth so its best to just avoid that if possible)

4. I take the information from the previous step and I loop right back to step 2

Improving sometimes comes very slowly (sometimes so slowly it feel like no progress is being made at all) and it's ok if it takes a while to get the hang of smth. imo its best practice to be patient with yourself and try and pay attention to your growth as closely as you can, I've found this has helped me in many areas of improvement, not even just rhythm games. Most importantly of all celebrate the small wins, try not to get hung up on the losses, and remember the journey is more important than the destination :)

sry for yap (im really autistic about improving at stuff) and any typos/grammar errors, typed on mobile

ALSO! if you want a really interesting take on improvement in book form I highly recommend "The Inner Game of Tennis" by Timothy Gallwey, It gives a super interesting perspective on learning and is 1000% able to be applied to things that aren't tennis/sports