It's hard. It sounds horribly twee but I found the only way to get through it was to appreciate what I had now & take each day as it came, not waste a moment of the time we did have wishing for what I didn't have any more. Tennyson had it right with "Though much is taken, much abides; and though We are not now that strength which in old days Moved earth and heaven, that which we are, we are" & maybe a touch of Van Dyke too "And what prayers For the silent strength that nerves us to endure Things we cannot cure!".
Your vets haven't given up hope yet, they still think he can return to some form of work, and nowadays I find vets tend to be pessimistic in a prognosis to protect themselves. He may not come right, but for now you still have him & that's a lot t be thankful for.