I know exactly how you feel, man. I just had my dog get run over a few weeks ago. I still break down too every now and then.
It's OK to feel sad, but your cat isn't in pain anymore, and he's most likely in a very good place right now. Whatever you do, don't blame yourself. What's done is done. All you can do now is never forget him and all the great memories you have with him.
Good luck coping. I feel for you.
ZJ
Sorry for your loss.