This is right up my alley. When I first started having panic attacks this time last year, I literally couldn't even power walk any more or walk up a flight of stairs too quickly because of the increase in heart rate. And this coming from some one who was huge into weight lifting and running before my panic attacks. I couldn't even lift weights any more. I used to deadlift 405 and then I tried starting small to get over the fear with 95 pounds which for me feels like a feather, I remember picking it up and had such an intense panic attack after I called my mom and told her to stay on the phone with me as I left the gym and drove back home. I'm 98% better in terms of exersice anxiety now. I think the way I got over it is baby steps but the key is to face the fear!!! No matter how much panic it gives you or fast your heart is pumping just stand there take a deep breath and accept that your heart is racing and that it might not slow down. I started going to the gym with my mom, then by myself but lifted very light weight, then I started jogging but at a very slow pace. I constantly had panic attacks and even cut my workouts short at times because of the panic. But I kept doing it, rationalizing your fear helps too but not to the extend as just doing it and trying as hard as you can to accept the uncomfortable sensations. I started lifting heavy weight again and even started sprinting, and I would still constantly think about how this might affect me negatively and cause some anxiety but I was obviously doing much better because I was actually lifting heavy and running fast. After I would say 9 months of consistently facing my fears, I feel no worries about excersise any more. I wouldn't say I'm 100% like the way I was before my first panic attack but BELIEVE me when I say I'm 100 times better now. I do max reps with really heavy weight all the time now and don't even think or notice my heart rate any more. Eventually once you get accustomed to the anxiety the last step is that naturally your focus changes because of how little you care.