Sharp pain under left breast and sternum, have had anxiety attacks before not sure if it is serious or not. i'm 21?

Panic attack?: Anxiety and panic attacks can cause many physical symptoms including chest pain and difficulty breathing. However, if you've not had a full medical evaluation for this pain, you need one even though you're only 21. If it is anxiety, there are good treatments -- both with and without medications. If you have another medical condition contributing to your symptoms, your doctor can help that too.

Anxiety.: Consult with your md to rule out physical causes. If all ok i'd suggest seeing a mental health professional who can help you deal with your issues.