Hardrock increases the overall difficulty and the approach rate of the map so it requires extra practice. I would recommend Beatmaps like t/136834&start=0 (Lunatic), http://osu.ppy.sh/b/46587 (Hanahubuki) and http://osu.ppy.sh/b/65855 (Rolling S i R i R u). The first and foremost step of learning HR is getting used to AR10. Then work on the accuracy.
Keep playing songs! Almost everyone gets to that point where they think they're not improving but you are! You have good days and bad days. Some days you subconsciously have high expectations and reverse to that. Have fun and keep moving.