Healing isn't necessarily difficult, but there IS a drastic change in awareness required when you reach something that's not a faceroll. I find that once I've fallen into autopilot snooze mode, anything that's a challenge suddenly becomes much harder than it should be, and I'm really not sure how to combat this.
It's not just healing mechanics that get fumbled either, just shoddy play in general. Kiting on Garalon, positioning on Bladelord, controlling adds on Elegon, timing heals on Tsulong. I can handle all those things, really!
Would it help to deliberately handicap myself on the stupidly easy stuff, like doing Spirit Kings with no weapon equipped? Take a break and play QWOP for a few minutes to clear the easymode haze out of my head before doing the bosses that are actually meaningful? Megadose on caffeine so I'm physically incapable of zoning out?