Sorry to hear what you're going through - panic attacks and anxiety are definitely unpleasant (to say the least), and the physical symptoms combined with the emotions and fear can create a horrible cycle. I have been experiencing anxiety and panic attacks since I was a child, and get a lot of physical symptoms (i.e. palpitations, shortness of breath, tremors, chest pain). They more I focus on them, the worse they get. I'm glad you have seen a doctor and had some tests done, as it is important to make sure there isn't anything physical going on that that needs to be treated differently.
A few things I have found helpful - deep breathing and meditation. I'm sure you've been told this, but it definitely helps. Even 1 minute of silent meditation a day can make a huge difference in helping to prevent the anxiety from building up during the course of a day, and help you learn mindfulness for when anxiety does kick up a notch. When I find I start getting palpitations, shortness of breath etc. that's a trigger for me to go and find a quiet place to sit, close my eyes and take some deep breaths in and out. I have done this at work many times, even if it means going to the bathroom for a short period to get away from excess noise/people. If that doesn't help, I listen to music - loudly - and try to go for a walk or get out of whatever environment I'm in. Outdoor walks are helpful as there's a lot to focus on (sounds, sights) and less immediate triggers. Speaking of which; try to pay attention to what;s happening to trigger your anxiety - who are you around when it happens? What's happening? Prevention is truely key.
I hope all that helps.