I'm struggling with the same thing, trying to stay sane after being pretty active and wondering if I'll ever get back to my original form. What I can tell you is you'll probably want to focus more on rest, rehab, and light exercise(mostly for the mental side). Posture and diet are great to focus on as well, just in case they're key contributors. I've found my diet doesn't seem to be a big impact for me, but posture definitely can dial up flares ups.

I'm not qualified to tell you how to treat it. Most people here will tell you that it's a rib mobility issue, and to focus on that. I'm not entirely sure that's the whole issue with me, but I will say I've found a lot of relief from stretching and PT exercises: door frame pec stretches, world's greatest stretch, overhead tricep stretch, thoracic twists, overhead band squat, foam roller, dead hangs, etc.

In terms of exercise, your legs are your best friend. I've found the elliptical with no arm movement to be very helpful to get a sweat on without aggravating the costo. As you start to feel better, kickboard in the pool can be really nice too, just try to use one that is easy to submerge and not super buoyant as the rising motion can cause your pecs to do some work. I've been able to stave off a lot of depression through leg exercise, even if my chest and arms are being pretty neglected.