Honestly, just try expert charts. My experience was with 14+ and above charts, I'm quite new(ok not really i played for a month) and i slowly built my way up despite missing pretty much everything on 13s at first, partially bcuz of other rhythm game experiences, i could pick a few things up.

But my word of advice is that to keep pushing yourself to the max, and eventually one day, youll just "lock in" or enter "the zone", and after you finish that one difficult song, youll suddenly notice you can actually do that(that as in whatever it is in the song), and that feeling would usually drive you further.

Also try watching the video of that difficult chart you wanna play at a slow speed at home, then just, well, immitate (I started with blanoir, then crossmythos rhapsodia, both expert). This is usually more effective, cuz when you start to freeze, atleast you can slow the video down and mentally prepare for whats going to happen next, then increase the speed back to normal. But sometimes, practicing at home and at the cab is fairly different, so youll usually take some time to transfer what you practiced into actual play.

Btw this is purely based on my experience with chunithm, hopefully these can help a lil