Yes try not to beat yourself up about it, easier said than done, but as everyone has said cats are very good at hiding their pain,also unlike us humans they don't think about it.
A good idea would be to get a kitty electric blanket if you can, i have two of them, and they are great for cats with arthritis, being a sufferer myself i know that warmth is a top priority and is really helpful.

I am really sorry to read about Max,enjoy your boy as much as possible while he is still with you,now he had meds I am sure he will be much happier.

Love and Hugs to you both.