i am so sorry for your loss and please don't feel guilty. i went through the exact same thing as you 2 weeks today and my bunny was right as rain the day before and it all happened very fast, withing 24 hours she had passed away. You couldn't have done anythjing to prevent it as you were obviously taking very good care of him. My bunny made it through the op but sadly died shortly afterwards so i know how devastated you are but you tried.
It's so unfair to lose a bunny this way and i hope in a few weeks, like me, you will feel a little bit better and will overcome the shock,
hugs
Lisa