But for the grace of God there go I means that if it weren't for God's grace (His goodness to me) I might be headed down the same bad path that someone else has gone down. It's not usually used to lament a death by natural causes. But it could be used to lament the death of someone doing something foolish/destructive that resulted in their death.
Ex: He had to go into rehab for his drug addiction. But for the grace of God there go I.
1. Lost time never comes back.
2. Last night is better than saying yesterday night.
This part does not make sense: but it slipped his mind - there but for the grace of God go I!
What slipped his mind? What are you wanting to say here?