Single faucet in bathtub loses pressure when turned to hot

Hello, maybe this has been posted before. I just moved into a house with a wonderful garden tub. It has a single faucet and when you first turn it on, everything is fine. However, as you turn it more to the left to get to warm and then hot water, the pressure is reduced to a trickle. Do I need to call someone? Please help . . . I'm stressed out and need a hot bubble bath.

First suggestion is that you have a pressure balanced type valve, and the balancing spool is stuck. It can be lubricated, or often just replaced. If you post a pic, first of the whole thing, and then a pic of the valve with handle removed, we can offer more advice on how to proceed. What brand?