Yes, I have been through all that, too, and was diagnosed with Vasculitis 3 years ago. I have remained on the prednisone and methotrexate since then, too. Sometimes at higher doses, sometimes down to 20 or 25 mg per day (pred). I have terrible circulation in my hands, legs, intestines, etc. I get chest pain walking up stairs and my legs ache just walking a couple of blocks. That sound like what you have been going through?
Thing of it is, we cannot give up on physical activity entirely. We are very, very limited and have to be careful (the doctors tell me I am an extreme risk of heart attack), but we still need to keep the flow going as best as we can. Even if all I do is 5 minutes of lite exercise (toe touches or something like that), I get the body moving. The people here got me going, actually. I have progressed from the wheelchair to walking some since I started the little exercise regime. And I actually got out in the yard (in the shade) and scooted around on my bottom to work in my flower beds over the weekend. That was the first time in 3 years!
I know how scary the diagnosis is, and the limitations are extremely frustrating. If you need to talk, please PM me, and I will be happy to listen. But the people here are great to listen, too.
Good luck to you.