I have constant tachycardia (although maybe not as fast as yours) for a year. It was faster at first but has gone down slightly. I'm now taking Propranolol (beta blocker) and it's helped but it still goes up when I get anxious. I know what your going through, it can be a real bother. It started all of a sudden for me, but the fact that I had mercury poisoning couldn't have helped either..