If you can not turn off the outside shower, and only shutting off at the head, it is cross feeding the hot and the cold. Get the water pressure up, water heater full, and hot, and then go to the outside shower. Open it, and let water run, and after a minute, check the temp. If it is warm, turn the cold valve the other way, all the way. Then wait a minute and turn see if it gets warmer or colder. If warmer, turn the hot valve all the way, and see if it will stop flowing, if colder, turn the cold back all the way, and then turn thr hot all the way, and see if it will stop flowing. Both valves have an "off" position, you just have to find them. As long as you are shutting off at the shower head, water can "cross flow", and you will always have a problem.
Just move one valve, one way, and wait for temp change. Takes a little while on outside showers, depending on length of pipe used to get there.
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