That's funny cause i suck at downstrock and upstrock i found it pretty easy, idk why but over the years i kinda get the habbit of starting some riff with upstrock, thats some bad habbit we need to get rid of and its pretty when its comin instinctively.
Anyway for your issue, what you probably do for downstrock, ur holding ur pick at ~45° angle with the strings so the pick just glide on the strings on the way down, try to twist ur wrist so ur pick is at the same angle when going up, so it will glide easy, if you allready do this right, a good method to learn the basic of sweep is by using a metronome and block all the strings so they're muted and practice goin up and down and get a very clean and timed sweep motion, start very slow and build up as you get confortable, your brains record the motion when you doin it slow, with time you'll have a good sweep glide at higher speed without even thinking,
then for your left hand, just practice the arpeggios without picking, and then you mix both of the things u just learn, and should improve pretty good,
Hope it helps