If your pressure problem was only in the shower, I would suspect the shower head may have trash in it reducing the flow.Since your problem is in both the shower and the tub, I wonder if there is not some loostened mineral deposits or other trash which have lodged in the valve. I would turn off your water supply to the tub and disassemble the valve. Get someone to hold a container over the open valve and turn on the water supply very briefly. This may blow an obstruction out of the valve.