what can cause a breathing SOB/ even when laying down problem & have had all the tests for heart & lung (all positively clear) & not anxiety

Doctors not giving me a diagnosis!!! desperate!!! not imagining this, am SOB on exertion, get heart flipping, throbbing & pulsating in chest & throat am felling all of this while on the computer, this is not attacks it is 24/7!!! any ideas !!!