Changing your playstyle feels difficult at at first and often leads to lower scores for a little while, until you adjust, but after the adjustment period, it's common to see a quick jump in skill.
Playing stuff you B or C is a great way to begin to push your skill ceiling up. I like to think of it this way: Let's say you can get ~95% acc on a 3 star mania map, but can only C a 4 star map (these numbers are only to get my point across). If you push your skill cap to where you can maybe get a mid B on that 4 star map, you might find that now you can get ~95% on a 3.5 star map, and a slightly higher acc on the 3 star map.
Making that jump from ~95 to 100% is incredibly hard. (case in point, I still don't have any ranked SS's in mania)