Complex sentence: Pain behind the sternum (breastbone) or in the epigastric region around the stomach, often of a burning, heartburn-like nature, may be frequent and severe, worsening when food of any sort is swallowed.
Split into simpler sentences: Pain behind the sternum (breastbone) or in the epigastric region around the stomach often feels like heartburn. The pain can frequently be severe, worsening when food of any sort is swallowed.