Shower hot water

I have a water on demand system I put in about a year ago. My problem is my system is an old one for the tub and shower. It has a hot water knob and a cold water knob with a knob in the middle to change from tub to shower. I'm getting real hot watering every other area of the house,kitchen sink and two bathroom sinks. Also washing machine is hot. The shower is now only giving me luke warm temperatures. I changed shower head but same thing happened . What could be causing this?
Thank you for taking the time out to read this.
Wayne

Sounds like it’s time to get some tools out. Take off the trim plate and see if you have integral stops in your shower. If not shut off water and replace shower cartridge.