hi and welcome. well smoking
can affect lungs that can cause pulmonary hypertension
and thus can cause right atrial abnormaliies , not left.
with left atrial abnormalities suggested by the ekg... the likely cause esp in females is MITRAL VALVE DISEASE
but your exertion capacity contradicts it. well if you feel that your exercise capacity is good but is declining then its ideal to get an ECHO of the heart and left atrium can be visualized and assessed.
take care and do get a check from a cardiologist with a repeat ekg .