I keep thinking I'm going to have a heart attack, would a ECG show up a problem as it comes back normal so is blood pressure I'm fed up of anxiety?

Why? Stop smoking. Check cholesterol get a cardiovascular rehab/ or exercize program going. May need a treadmill first as screening control weight check family history. Ekg does not predict but can be helpful if having arrythmia or chest pain. See your pcp and work on the causes of your anxiety! also.