Exercise intolerance, how to deal with it?

After getting my heart checked once again with a stress test, 14 day monitor, 24h pressure cuff, blood test and orthostatic test, I’ve been told my heart is alright and it’s not the cause of my sudden exercise intolerance (started experiencing this last December). Discussed so many things with the cardiologist that I forgot to actually ask her the most important question: if it’s not the heart, what is the cause of my symptoms? My heart rate is constantly elevated but according to my monitor my avg was around 90s (not very active), and my stress test gave me a MTE of 7.3 and a “excellent tolerance to exercise”, while also noting that the test was “terminated secondary to dyspnea and fatigue” (incline walking and power of walking, 0 jogging or running) with a maximum heart rate of 176bpm. My concerns were autonomic problems if I didn’t have anything wrong with my heart but apparently the BP is nothing crazy, so the cardiologist doubts it’s related to POTS/IST. What should I do now? I miss working out and can’t really imagine a life beyond were it only takes me to walk around for my heart rate to be 120-130bpm.