How can I convince myself I'm ok?

I've been severely anxious for 6 months now. My #1 symptom is tachycardia and a feeling of inevitable death by heart failure. Thanks to this I've had 3 EKG's, and echocardiogram, chest x-rays, blood tests and urine tests done. I've also visited three different cardiologists who have all asured me I'm fine. After all I'm a healthy, non smoker 19 year old with no family history of heart failure. I just started setraline a week ago, but my mind continues telling me there's something wrong with my health. What can I do to convince myself otherwise?