Your Chewie was adorable. I am so sorry. I had tears reading his story. There is nothing anyone can say at the moment to ease your grief, and along with that grief comes the guilt. They just go hand in hand, regardless. Everyone will say you must not take on the guilt because have nothing to feel guilty about, but I know from experience it doesn't work that way. It seems Chewie was quite poorly, and you were with him and loved him to the end. That is all you could do.
When I lost my Timothy I hit the internet to read every article I could about coping with this type of grief and guilt. Somewhere, anywhere, I wanted to read those words that would magically take away the crushing heartache. Of course they aren't there. I learnt I would have to get through it in my own way. The only words of comfort I can give you is that, although you honestly never think it will, the grief and guilt does get easier to deal with in time. It's been three months since I lost my Timothy. I still cry everyday and I still feel pangs of guilt that keep me awake at night, but it's not overwhelming. It's just a new 'normal' that no one wants.
Chewie was young, but he knew you loved him.
Thinking of you
" The pain I feel now is the happiness I had before. That's the deal" C.S. Lewis