Hello, I (31M, UK) recently had a diagnosis of "moderate pectus excavatum" but noting "that it's less severe than the Haller index indicates as the whole chest is relatively flat" (platythorax?), and that my heart has adjusted well to having less space, concluding that this is not the cause of my symptoms. The HI and attached scan were measured on exhale.

I've suffered from fatigue, low energy, and poor exercise tolerance for as long as I can remember. About 60 seconds of a light jog is enough to get my heart rate from 80-90bpm to over 180bpm and have me out of breath, and going over 200bpm isn't unusual either. Bloods and other tests I've had always come back fine, and my build seems okay. (173cm, 63kg), so there's no other obvious causes.

For exercise, I've done martial arts for years, but I've always had the worst stamina in the room by a very wide margin and feel like I'm operating at my limit almost all the time. Literally everyone else seems to find it so much easier, regardless of age, experience, or any other health conditions.

The only advice I've been given is "do more exercise to improve your stroke volume", which doesn't seem right given I'm already fairly active. And it wasn't really explained to me how this is supposedly harmless and not having a negative impact on my heart.  
  
So I wanted to ask if anyone had any opinions on this? 

Haller index 5, but doctors say it's fine?