You will feel a lot better after the ercp. I went through the same thing a few months ago and my bile duct was completely blocked when I came in. I was in a lot of pain even the same day the ercp was done, and afterwards it was as if the pain never had been there.
I've also had pancreatitis as a side effect of the stone in the bile duct and yes, you lose a lot of weight and get tired. Your liver is not working very well when your bile duct is blocked and due to that you get tired and drained.
Good luck with the mri and I hope the surgery will solve things for you.
C.