How serious is Tietze syndrome?

It seems like you are addressing several different complaints in your question. Tietze syndrome is a rare form of isolated inflammation involving the joints between the sternum (breast bone), clavicle (collar bone), and ribs. However, this doesn't have anything to do with your pain in your feet. Furthermore Tietze syndrome is usually self resolving, and it would be unlikely for symptoms to continue for 3 years unabated. Finally, Tietze syndrome just causing inflammation of the joints, not the bones themselves, so it would be unlikely to cause your collar bone to stick out like you are reporting (this could actually just be your normal collarbone anatomy). I think you will need the help of your primary care doctor in order to help sort out what is going on, so I recommend that you touch base with your primary care doctor. There are multiple potential causes of chronic joint and musculoskeletal pain. These include conditions like fibromyalgia as well as numerous rheumatological conditions, like lupus. Your doctor will help you determine whether or not it makes sense to pursue a rheumatological workup or whether attention needs to start being focused on helping you control and manage this chronic pain problem.