Hi Groaner and welcome.
I too have problems with severe arm pain. Mine isn't constant, but is quite frequent. Sometimes it seems related to overdoing it, but sometimes it comes out of nowhere.
I also use heat and massage, but what helps the most is just keeping it very still for awhile (very inconvenient). I've even had to use a sling to keep the affected arm still while using only the other one.
I'm sorry you have to deal with such awful arm pain. It can be quite a trial.