Took ekg about 4 times, blood test twice, chest x-ray once, all normal but then tell me why?

i have been diagnosed w/ panic disorder, but somehow i still cannot shake off the feeling that something is wrong with my body. i am a borderline hypochondriac. ive been on lexapro for about two weeks now so the severity of the attacks has gotten a lot less intense, however i still have frequent episodes where my heart will feel like it is about to jump out of my chest. i also have an irregular rythm here and there where it feels like my heart missed a beat. it is so unpleasant and disturbing. ive been to the ER like 4 times in the past MONTH, they took an EKG, kept it on and monitered my heart for hours at a time, took blood enzyme tests twice, and took a chest xray once, docs said everything was absolutely normal but somehow i am still not convinced that this is all just because of panic disorder. how do i get all this to STOP???