I have a serious problem in one of my bathrooms. The cold water pressure of the shower/tub is fine. But when I turned the knob towards the hot, the pressure drops all of a sudden, too low to take a hot shower. The sink 3 feet away has both excellent hot and cold water pressure. I suspect some is wrong with the shower switch. I took the knob off and tried to see what's wrong, but I didn't see anything removable from that point on.

Everywhere else in the house is good.
Please help!

Thank you