Feeling guilt in this situation is understandable, but sometimes hard decisions need to be made. Although your mother (I'm guessing this is true) did not want the operation, it seems that it was needed, you tried your best and that's all you could have done. Do not feel guilty for what you couldn't do, be proud of what you could do, and be happy of the memories you have with your mother, even though it may be hard. Hang in there