It's just a really hard game. If you keep at it, it'll click for you eventually. I never finished it myself. Bit tough for my taste.
I tried numerous times and I couldn't get into it. It's not because it's hard, because I love challenging games. Games like System Shock 2, Shinobi for the PS2, Contra, Demon's Souls (which is harder than Dark Souls), Fallout 2, Devil May Cry 3, Cuphead, and Ninja Gaiden Black. These games were difficult, but often fair. The took wit and skill to play. Whereas Ninja Gaiden 2 is hard, but it's not fair at all. And I'm always perplexed it's regarded at such high regard. Maybe I'm just awful at it.