I agree that it's hard to watch an animal suffer. Although I think most times, animals only suffer because we, as pet lovers, selfishly keep them around because we don't want to lose them. I am not saying that is what happened in your case, it's just an observation I have made too many times.

It is hard, but I look at my pets as the huge joys I have in the present, and the good memories I will have when they are gone. Just give Boo and big hug, and thank him for blessing your life while he can, and for giving you memories that you will cherish forever - memories you wouldn't have if you never had him at all.