Sorry to hear of Keira's diagnosis.
Have you checked all of your policy documentation, to see if there's any way that they could have misled you (ie. to expect a lifelong cover)? Unfortunately it will probably say in the small print that it's limited for a year.
No insurer will cover for a pre-existing condition, as far as I'm aware. I'm in the same boat with my dog Henry as I had to change insurer when Axa put my premiums up by 610%! Hopefully he won't need an operation on a congenital cataract, because now we have to pay for it.