its not your fault though you DID do everything you could with the last few weeks/months he had with you, there is no way you could of forseen this, you didnt let him suffer anymore, hes in a pain free place now (hugs)
Oh Liz, he doesnt need you to say sorry, he will be glad of the love you gave him and he knows you tried to get him better.
Was just time for him to be a bunny Angel xx