I have been through something very similar. The world's briefest summary: was a dancer, got fibromyalgia, gained medication-related weight, found cycling, lost some but not all of the weight, then diagnosed in addition with mild RA (which has stayed mild. Still trying to get off the rest of that weight).
You learn to adjust your expectations, wants, needs, and activities according to the cycles these syndromes present to you. And actually, life's like this for many people for a wide variety of reasons. That doesn't make it fun, but learning to surf the rhythms of the ups and downs is a really useful coping skill to acquire, since we're all gonna need that skill at some point.
I think the best advice I have is, consider yourself in this for the long haul. You are going to have good periods and tough periods, but neither last, which is good to know. It won't be this bad forever. And other doors open (e.g., from ballet to triathlon). Keep an eye out for the other doors and consider taking some of them, since there's some pretty fun stuff in there that you probably hadn't noticed before.
Do a little research online to find others who have been through this. Believe me, they're out there--I looked too (google "runners with RA", etc.). Take a look at this blog: http://tribeyondlimits.blogspot.com/ I don't agree with everything she says, but she's a good example of what I mean about rolling with the ups and downs.
I hope this is a little helpful. It won't always feel this bad. It truly won't.
"My predominant feeling is one of gratitude. I have loved and been loved;I have been given much and I have given something in return...Above all, I have been a sentient being, a thinking animal, on this beautiful planet, and that in itself has been an enormous privilege and an adventure." O. Sacks