the problem with change up grips is getting the pads of the index and middle fingers off the ball(vern ruhle, major league pitching coach). everyone concentrates on the circle formed by the index finger, but get the pad of the middle finger off the ball and you should have some success. let the middle finger and index finger ride BESIDE THE BALL, not onthe ball. when you compete and leave these fingers on the bal, sometimes you will clamp down with them and that creates too much velocity.
think of throwing a fastball off the end of the ring finger. pedro gets his ring finger and thumb on the equator of the ball (you can see this on the slo-mo video in hitters on hitting finding the sweet spot video). relax your hand as much as possible and throw the thunder out of it.
my son actually throws his change up by throwing a two seam fastball grip on the narrow part of the seams using the ring and pinky fingers on the seams. sounds crazy but at 15 he just struck out 10with no runs in 6 innings against an elite 16 yr old team. the change is 15 mph below his fastball which is good but not great (he hit 85 once and pitches at 81-83). but that looks really fast when the change comes in at 70-68 with arm speed. that's the bugs bunny you hear about during the playoffs.
also, play long toss using your change up grip and throw it into the end of a cage 50 times every other day to develop feel. didn't say it was easy, just effective and deadly.
keep working. you'll get it if you want it bad enough.