Rachael, believe me, you're not the only one who has gone through this very same thing. When my Theodora was a young lass, she 'played so hard' (or so I thought) that she panted. She was 6 months old when I took her to the vet, who almost convinced me that it was because she wore herself out! When she started open-mouth breathing on the x-ray table, the vet realized it certainly was abnormal. More x-rays and an echocardiogram later, she was diagnosed with HCM. She'll be two years old next month, and she's finally not quite as playful (but, oh, how she misses her laser toy......). The vet said to "keep her calm" (impossible), but all I can do is interrupt her when she gets too active. I will let her enjoy her life and, for myself, I'll cherish every single moment I have with Her Orneryness. I can't imagine my life without her; all I hope is that if the good do, indeed, die young, then Bad Girl Theo will live forever.
Before you criticize someone, you should walk a mile in his shoes. That way, when you criticize him, you're a mile away and you have his shoes.