It's not too late. The enzyme tests measure substances that leak from damaged heart muscle. That continues for days or weeks after the event. The ECG trace is also distinctive. And it will make a HUGE difference if you can get them to find your previous ECG records - changes in the ECG are very significant, more so than the trace taken in isolation.

RA effects on the heart are different - usually valve damage. This leads to the heart working less efficiently (hence the chronic breathlessness), but doesn't have much to do with heart attack.

A doctor who makes a patient feel like a "hysterical woman" is a crap doctor. You deserve to be taken seriously.