I am so sorry for your loss.
Have a real good cry, don't hold back, and don't feel any shame, just let go and bawl your eyes out! This sounds like strange advice, when most people say be brave, or don't cry, you'll only upset yourself, I disagree.
The measure of any living creature is how much they were loved, and it sounds to me as if Casper was loved VERY much, so have a good cry for him, he deserves it, he was important enough for you to grieve for him, its not weakness, its the strength to show how much he was loved 8...8...
Then dry your eyes and blow your nose, you can grieve guilt free, you did EVERYTHING you could for him, and you made him feel loved in his last moments, no piggie or any other creature could ask for more.
Rest in Peace Casper, the pain will ease Mummy.x