Try unbuckling the buckle under your right arm, which will take the weight entirely off your left shoulder. Now the straps in use are nearly the same as the waist-plus-right-shoulder type of harness. Adjust it so you can wear the drum the way you would with a single shoulder+waist harness - with the lower D ring slid over to in front of the left hip and the waist very snug  Then adjust the left shoulder to match the right. 

Possibly also decreasing the length of the vertical section between the two D rings (see FAQ #2 below) can help as well.