Nothing can ruin a shower quicker than a sudden rush of cold water except a sudden rush of hot water. A sudden rush of scalding hot water in the shower can be dangerous. If your shower is prone to temperature fluctuations it may be time to install (or repair) a pressure balance shower valve. The hot and cold water regulation of a pressure valve can prevent scalding when water is used anywhere else in the house.
Don't put up with unreliable water temperature; it's not worth it. If you are not comfortable with installing a shower valve, hire a plumber to do the job. Don't forget to get estimates to make sure you are getting a competitive price. Ask for two price quotes: one for the plumber to provide and install the valve and one for just the install if you purchase the valve ahead of time.