This is a tough one. How about you start by writing the letter to yourself? Maybe just to sort out what you wish you would have done differently? Don't know, just an idea.
As far as timing, I'm kind of at a loss for advice. My apology came early and seemed to help. My H was feeling ignored and unheard, so maybe that is why it helped. Though it did help diffuse the tension, it didn't stop him from going ahead and moving out.
