Please stop beating your self up and putting yourself down. No one, and you included, deserved the treatment that we have been dealt by our spouses.
There is absolutely no way that you could have prevented what he's experiencing. This trip that they are on is all about themselves and growing up. We were not there to protect them when they were young....
As for you, this is a time for you to spread your wings and soar. Life is far, far too short to live in the past. Look to the future, for you will get there in time. Live in the present and know that life will be good again, in time.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
