There is this weird pressure on the middle of my sternum. it also extends into the left and right side of my upper chest. its higher up, about an inch or two under my clavicle. is it muscle or my heart? i'm 21?

It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Drink more water. Wish you good health!

Musculoskeletal rib cage causes are likely in your age group. Any recent strain or trauma? Usually there is tenderness or increased discomfort on motions like deep breaths,turning or twisting. Avoiding strain&taking anti-inflammatory meds like ibuprofen would help. Without cough,wheeze a tracheal or pulmonary problem is unlikely. Heartburn,swallowing symptoms would suggest esophageal cause.See MD