How can you fix a shower valve that has great pressure to the tub but very low pressure to the shower head?

First, remove the shower head and test the valve. If you have plenty of water at the shower with the head removed, then the problem is a clogged shower head. Clean it or replace it.

What the hell...just replace it. You deserve it. Go buy one of those cool shower massage heads to replace it.

If you discover you have poor flow at the head with the shower head removed, then you have a blockage in your valve.

So you buy a 1/2 (female pipe thread) x Hose Thread adapter. Put that on the arm for your shower head. Turn off the water to the house. Borrow a neighbors hose and run it through a window and connect it to the shower head (hose thread adapter). Now take the hose off your washer box and drop the hose into the drain hole. Now turn on the water.

The water from your neighbor will allow you to backflush that valve. The debris will backflush and wash out through your laundry hose on down the drain.

If that doesn't work, then yes, you probably need to replace the entire valve.

(I'm assuming your already taken the cartridge apart and cleaned it)

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Is your tub spout still running alot of water through it when you use the shower? If so replace spout, if three valve, replace center diverter. If that is all ok, Then yes take off shower head and see if you have the pressure, if so install new shower head and or clean. If not head, then screw on .5 x hose adpt. to the shower arm, you will have to have one washer hose to connect to your outside faucet then to your garden hose which will connect to the shower arm. Leave faucet hot and cold water off, turn on outside faucet all the way, this will let the water go through the valve and out the spout. If this does not work replace faucet.