I love exercising, but also tend to be very triggered by it when my heartrate increases beyond a certain rate as you mention, or when other physiological markers of extreme exertion kick in. I also tend to become obsessive and self-punishing in my use of strenuous exercise, and so it is an incredibly delicate balance for me to seek and maintain.
I have worked hard to try to find my coping threshold from a heartrate perspective and to keep my exertion at a level that is within my tolerable range, not triggering me too much but also challenging me just enough to slowly achieve an exposure effect over time. I was very lucky in that my therapist, who is also a fitness guru, has been very supportive and involved and has helped me to set and maintain exercise regimes and to monitor them with many of the exposure therapy principles that we have utilised in traditional trauma therapy.
Over time I have improved and have learned to tolerate the pain and physiological stress of exertion with less triggering, though it is still something I need to monitor, especially when abnormally stressed or anxious to begin with.
Have also done a lot of work with safe place imagery and holding soothing images in mind while exercising - a kind of action meditation (my term, nothing official...), and this too has gradually helped over time.
Obviously, where there are physiological issues such as a heart condition involved, it's important to monitor and discuss these with a doctor/cardiologist as a first step. I too have a heart condition, but for me the disturbance is mostly emotional/psychological.
Wishing you all the best, exercise is such a critical part of healing and coping for me and is something I am very passionate about, but am also very aware of its challenges, and so I deeply empathise with you.
Maddog