Why does my heat pump keep triggering the high-pressure cut-off?
This is a classic sign of low refrigerant. Refrigerant is not a consumable, so low levels indicate a problem. This is not something you can fix yourself. Call a technician, but don't let them just top it off and leave; make them stick around to find the leak.

Since this is a heat pump, the expansion valve is actually a mechanical device and not just a simple orifice like in a regular air conditioner. A failed expansion valve can cause the pressures to be incorrect, mimicking the problem of low refrigerant.