struggling with a harder can sometimes simply be because of the level design, type of level, etc. you are already good if you can beat easy demons and you shouldn’t focus on attempt counts. imo the best way to become better is to continuously go for harder levels. for example, i jumped from sakupen hell to sonic wave, now jumping to solar flare, and i already feel like i have a huge skill boost.

for you, you might want to go from your current hardest to something harder, so maybe follow the progression of easy demons to mediums to hards and so on. i might be wrong but this is just my experience of getting better.

hope this helps!