I have single handle pfister that I had put in. The plumber put in the cartridge and I put in the rest. I really like the way it looks but not how the shower head works because of the water pressure restrictor. When I unscrewed the shower head, I found an o-ring that was blocking this green thing with holes in it. I removed the o-ring and the pressure improved some but was still weak. I noticed that the green thing has 4 tabs that fit into hole of the washer blocking most of the water. What water that does goes though these little holes . I tried to remove it with small flathead but it won't budge. I'm thinking of getting out my drill but I don't want to damage the showerhead. How do I remove this thing?