Temperature control is either all hot or all cold. No in-between.

Hello,

It looks like you have Glacier Bay shower faucet. If that's the case then proceed as follows:

Remove handle and trim plate exposing two small screw-like Hot and Cold water valves. Turn them OFF with flat screw driver. Proceed with removing sleeve ( or nut ) that holds cartridge in. Pull on the cartridge with channel locks and remove it out of the body of the faucet. Replace cartridge with new one. Reassemble trim. Open water.

If you don't have close stops under the trim then you will have to turn off main water for the whole house. These days we don't replace individual O-rings in cartridges. Cartridges are inexpensive and easy to work with. Replacing cartridge will take care of all your possible problems you may have with your faucet.

Hope that helps

Milo