Sorry to hear that you are going through this. My only advice is if you know he is suffering to let him him/her go. I just went through this 3 months ago approximately and I am still heartbroken over it and it hasn't gotten any better at all.
I think what makes my situation so hard is that I got terrible medical advice from an emergency animal hospital (Med Vets Dayton Ohio) and that is why I chose to put him to sleep that awful night. The guilt is eating me up inside and now I have to live with it forever until the day that i die unfortunately. Buddy's passing is the worst thing that has ever happened to me in my life.
I really do wish you well in coping with this and im sorry that this is happening to you.