Severe pain & pressure in sternum. tried gaviscon and nsaids. had ekg 2 days ago & chest xray 4 days ago all clear. nothing is helping. terrified they missed something with my heart or lungs because i'm so uncomfortable what could this possibly be?

I assume there has been no trauma or strain. I assume no esophageal symptoms or GERd. Costochondritis, an inflammation at the junction of the rib & sternum is a possibility. The joint would be tender to touch & responds to rest,,heat applications and anti-inflammatory meds like advil. I've referred patients with your symptom to a thoracic surgeon for evaluation when other causes were ruled out.