Heather, Generally the ow is already waiting in the wings just before the BD. Until your h actually dropped the bomb on you, he wasn't in full blown mlc. What you may have witnessed was the "leading up" or clinical depression to mlc. It takes 18-24 months time prior to the BD for them to work up to crisis mode. They start to feel anxious, like something isn't quite right, but can't put their finger on it and then something happens in their lives that spins them up and sets them on their way for the mlc.
Keep in mind, mlcers can start out in emotional affairs that could lead to phyiscal affairs later on in replay. Either way, someone could have been in his life prior to the BD. There are some mlcers that never got involved w/an op and were just happy to be left alone to do whatever it is they need to do. Keep in mind, each mlcer is different, but may have similiar behaviors as others.
I may be wrong, but I don't think your h is on the downward slide of the mountain. He's just getting started and the insanity will take as long as it takes for him to work through his issues. The best thing you can do is buckle up and be ready for the ride.
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.
