My heater only works when I am going a constant high speed on the freeway.
If the blower fan is operational, then you might have a clogged cabin air filter. The filter is located behind the glovebox and should be changed periodically, to ensure proper HVAC operation and air volume. If the blower is not operating, then the blower motor circuit and relay may need to be diagnosed. You may want to enlist the help of a mechanic, such as one from YourMechanic, who can check the air volume of your HVAC system, and diagnose and repair the low air issue.