Has anyone mentioned costicondritis?
I have HOCM and my daughter had same symptoms you describe, so of course we thought was her heart also. We went through battery of tests you mentioned also. .Ended up not being the heart at all. She had pain in center of chest like you describe which in turn lead to anxiety attacks ( she was 17 at time). Anxiety attacks came on because of severe pain, which feels like you are having a heart attack. Adults experience the same sensations.
I am not trying to downplay your condition at all, and as a mother I am happy nothing was wrong with your heart. There is nothing more frustrating and frightening, when you are young and feel like your heart is racing and you experience pain.
I would ask Dr about costicondritis and also ask him to do Lyme tests- not regular ones only- ask him/her to test for babesiosis. Undiagnosed, this can lead to same symptoms.
Just some suggestions from a Mom!
Hope you figure out and that there is nothing serious going on!