I'm so sorry for your loss. I think we all know that cats are more than just pets, they are best friends, and family members, and losing them just hurts as badly. Please do not blame yourself. You had no way of knowing he was in renal failure. Cats are very adept at hiding their pain and discomfort. If he was in such severe renal failure, it's likely the outcome would have been the same if you brought him in the previous day. He was in pain and suffering, and you made the compassionate decision to end that pain. This isn't you're fault, there is no wrong/right here. It's just a very sad part of being a pet parent. Sometimes we only have them for a couple of years, sometimes we have them for 20, but they will all eventually leave us.
When I lost my first cat who was 19 (I got her when I was 2, and was 21 when she died, so she'd been my best friend, literally my entire life), I took a week off of work. Everyone grieves differently. You're definitely not crazy for being upset. Eventually, the pain will pass and you'll be able to remember the happy times with Bongo and all of the joy he brought to your life and vice versa.