Heart anxiety is fairly common. I have dealt with it on and off since I was a kid, and still do to this day. The Anxiety Guru has a free ebook on the subject that I found very helpful. But consider this: you're 20 years old! So it's incredibly unlikely that you have heart disease. And you've had lots of EKGs and other tests that haven't shown anything. If you had some kind of heart issue, it would have been detected by now. Resolve today to stop worrying about your heart. It knows what it's doing and will take care of you. Eat healthy foods, don't smoke, get exercise, and you'll be fine.
I had the same issue and the first thing the doctor did when I told him "i can feel my heart beat sometimes really hard and fast" ..he sent me to get my heart and thyroid levels checked. I ended up having heart palpitations and hyperactive thyroid.
Yes, it could be anxiety. All these strange feelings is all in our mind, try ignoring it. Do exercise daily and eat healthy.
Try meditation and healthy eating, take care of your health and do exercise everyday. Take proper medication and therapy to get rid of anxiety issues.