If your two EKGs came back normal, you don't have underlying heart disease, you've experienced your concerns to health professionals, and you know you're young, you need to find a way to accept that, yes, it is all just anxiety.
If you haven't looked up all the possible symptoms you can experience with anxiety that relate to your heart, my suggestion is to do it. Keep a list even. Part of the reason for that is that a new sensation you might feel is less likely to freak you out when you already know it's possible to experience it. You may drive yourself crazy otherwise or fall in the trap of a 'what if' loop.
Our heart is probably the scariest part of our body in terms of things going wrong. It's because we know what happens with a heart attack is often fatal, for instance. Definitely scarier than a stubbed pinky toe, right? Who ever died from that? So, what's going on is both physiological and psychological here. Physiological because heart palpitations occur, either as rapid heart beats or minor disturbances in your heart's rhythm, e.g 'flops' or 'skipping a beat'. Even without anxiety, those symptoms are common in normal, healthy individuals. I can tell you I get them every now and then too while experiencing no anxiety.
The issue is psychological in that you've turned your attention to your physical symptoms instead of learning to cope with a situation that is causing you distress. When you are so sensitised by anxiety, all it takes it is the tiniest sub-conscious trigger that we might not even realise happens, and not necessarily a huge freaking bus about to run us over.
Our brains tend to scan for danger and threat, and when nothing else seems wrong, that's when we notice our vital and important heart instead of pinky toes. So we think our heart is "pounding too hard" or "beating too fast". We immediately jump to conclusion of "yes! we've found the problem! There's nothing else wrong, it MUST be our heart that's the problem." Well, of course it's the problem when you've completely internalised the search for threats. Each time, learning to become hyper-vigilant, we get better and better (and lazier - our minds are great at cutting corners) at jumping to this conclusion. We even ignore that the solution may be as simple as just breathing slower.
How do you tackle this? Well, what you really want is to desensitise yourself of your fear about dying, which really is a normal and healthy fear to have until it starts creating blocks in your life. First off, don't underestimate or write-off the power of exercise. Doing it daily, such as walking helps to not only regulate your energy levels but it naturally decreases your heart rate (making it healthier, stronger) while also boosting 'feel-good' chemicals in your body that make you less anxious. Secondly, while it's scary that anxiety symptoms can resemble those of a heart attack, you want to learn resolve yourself and be OK with dying and/or having a heart attack. The physical symptoms highlighting your discomfort are just a psychological manifestation of your belief that you "can't handle it". But why couldn't you? What proof have you got that makes you certain of your own inability to handle the situation? The answer is something that'll have to come about through CBT or your own journey of learning.
Rest assured, your heart is strong (squeezing the equivalent of 100,000 tennis balls every day!), and you as a whole are stronger than your symptoms of anxiety.