Pick up a lower flex stick and try with that. I got a $20 Easton Classic intermediate woodie with a 70(?) flex and suddenly my slapshots improved alot, way more consistent accuracy and power. Down from 85 & 90.
Even if it's an in between step to going back to higher flex sticks, it's worth it. More forgiving, atleast until you get stronger with the shot, improve technique, or just get more confident.
For your bottom hand, it is easy to have it to low, but for max power it should be halfway down the shaft (give or take an inch or two) I have tested it, and that's how I get the most power using my radar gun.
This video is in the links above, but I figured I would just embed to make it easier.