Heat pumps all have defrost cycles where it'll run like a regular air conditioner to heat up the outdoor coils temporarily to melt off ice build up. It turns on

Heat pumps all have defrost cycles where it'll run like a regular air conditioner to heat up the outdoor coils temporarily to melt off ice build up. It turns on auxiliary electric heating inside in order to maintain heating while it does this. Yours clearly is broken in some form or another, it's either not going into a defrost cycle at all or even a full defrost cycle isn't enough to melt all of the ice and it's building up faster than it can be defrosted. Just call an HVAC company and get it fixed.