BIG HUGS
You have made absolutely the right decision, but it is going to take some time until your heart has caught up with your head again. We all suffer from feelings of guilt to some degree or other at the onset of the grieving process and mull over what has just happen/question our actions. This stronger after pts, a sudden unexpected death, a long drawn out battle and of course an accident or after making a clearly wrong decision. It only means that you are very loving and responsible owner.
Be kind to yourself in the coming days. The kidney failure is not your fault but a very short genetic straw that Austin has drawn. You have given him a happy and good life and have made the time he had to live worth living and his end as kind as possible. However long or short a life, these are the best and most important gifts you can make a pet of yours. You haven't failed him in any way!
If Austin has a companion, then here is what you can do for them:
Looking After A Bereaved Guinea Pig