It's very early days for you, and it's very natural to blame yourself. Not because you did anything wrong at all, but because guilt and blame are parts of the grieving process.
You need to try and take each moment at a time. Try drawing some comfort from your other buns. It can't replace Chance, but it can help you feel comfort to be near another friend. Having said that, I understand why it can be scary to love someone else and risk losing them. Love is always a risk, but hopefully with time it will seem like less of a risk as you start to heal from your grief.
Chance had a really good life with you, he was deeply loved and you did everything you can for him. Nothing will ever bring him back, but he will always stay with you in your memories and heart.
Hang in there.