Listen to the music, tap to the music.
HD is mostly just remembering where the hit circle kind of was, you just get the feel for it after you play it for a while.
Slap on the mod and play a few maps. The first few maps will be horrifying, leaving you think you just can't play it, as you described.
However, just bear with it. Keep playing, that's how you get better at it.
Higher AR makes HD easier as the effect of HD is less noticable. Reverse engineering that sentence also leads to the conclusion that the lower the AR, the harder it is to play with HD as the effects of it will be more present.
Don't really know how to explain it otherwise, it's how I learned it.