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Shower water issues - water is either too hot or too cold

I am seeking someone to fix our shower water issues please. It is very difficult to get the right shower water temperature, and often end up having showers that are either scalding hot or freezing cold. Not sure if this is thermostat issue, shower faucet, showerhead or something completely different. Need a plumber to get sorted asap please as we have young kids and elderly struggling with this issue.