What makes them feel as if you are slipping away? To me, that would be an individual answer for an MLCer. Some might care if you are "slipping away". Some might not, as it could alleviate their guilt. Some may wake up one morning and think "Wait. How did I get here?" Some may never do that.
It could be getting the divorce, or a new bf/gf, or moving away, or getting a job where you can support yourself and no longer "need" him, or stopping contact or selling the house or...or...or who knows.
If anyone had the true and definitive answer, many of us would not even be here because we'd have used it and be on the path to reconciliation if we wanted it.
What makes ME feel like I am slipping away? Detachment. Taking care of me. Making sure I will be ok on my own. Knowing that whatever life sends me, I'm going take it on, deal with it like an adult, not blame someone else for what i can't handle, be the best person I want to be, and not walk over anyone to get where I need to go.
Whether XH thinks I'm "slipping away" is a moot point. I'm not living my life for him. He gets to live his own life and reap what he has sown. Lost or not, MY H is gone at this time. He may or may not ever turn up, or someone else may appear in his skin suit. If the skin suit turns up at my door, I'll figure out if I like him then. If someone else turns up first, I'm ok with that, too. And I'm ok alone.