I Am Losing Hot Water, Is It My Water Heater?

I seem to lose hot water as I shower. My burner man says I have plenty of hot water and should test the shower mixer. Can the mixer be my problem? I've talked to some plumbers and they say I probably need a water heater.

Do not replace the water heater without first checking out the pressure balance faucet in the shower. Pressure balance and temperature control shower faucets are notorious for reducing the hot water pressure as you are using the faucet. Temperature control shower faucets use a spring that adjusts the hot water temperature down when it senses a change in the water temperature. Unfortunately when the spring gets old, it doesn't function as it was intended and you lose hot water temperature.