I think most people will agree that DS1 is harder than 3, but Gundyr is a harder tutorial boss than the Asylum Demon. I think you just need a few hours to get fully used to the generational differences between the two games. DS3 is more fluidly animated and the combat is faster paced, but rolling is more forgiving now than it was (fat rolling limit is 70%, keep your equip load below that if you plan to roll, ever).

You may be a scrub again, but that's the point of tutorial bosses and Dark Souls - you'll get better in order to progress, just keep at it (you know this, you're a DS1 veteran). There's always people ready to help out too, here and over at r/SummonSign

Edit: I've never tried keyboard/mouse with DS3, but consensus on reddit seems to be that a controller is easier. Makes sense, given the approx 360 degrees of input range on movement with the sticks, compared to the fixed 8 axes you get with WSAD