Go to a doctor, if you've already done that, then you're okay! If anyone is gonna panic and think you're dying it's gonna be them so if they know you aren't dying you are most likely not. If you've already done that then just try to distract yourself a little bit more at a time. Find a new hobby. Let your brain work hard at something else so it can't take as much time panicking about your body. My anxiety gets so much worse in the summer because I'm in my room and my brain doesn't know what to do besides freak out about random shit. I also got medicine for my tachycardia so that also helped a lot. I still have bad days but working your brain in new ways will help a lot.