So sorry for your loss. Like you said it was God's will, so for you to think it could have been prevented and taking partial blame for what happened is not good for you. I know it's easy for others to say but most of us have been there, myself included. There's always a reason for tragedy, although we may not like it and may not see it immediately, God has something else planned so just keep the faith!
"Dream big, Work hard and Have FAITH"
"When my story's told, I hope he'll say well done and welcome home." ~ Templeton Thompson