I had flowering heather/heath reccomended to me by a plant store person since they handle the acidity found under pine trees nicely, being an evergreen themself. I trimmed off the lower branches of my white pine and big spruce to let in enough light that I think it will qualify as partial sun and plan on getting them later when I have money available.

your problem is likely more water related than shade....pachysandra,vinca,ivy....are a few of the tough plants that offer more reliable production in tough areas