How do you navigate a partner pulling away or planning to leave while you still love them?
Love is a bond we form with people, and like any bond, it can be broken. It might take longer than you want, and it'll be painful, but with time and effort, it will happen.
Find ways to occupy your mind - new hobbies, go out with friends, live your life as if you're single. I do NOT recommend dating or moving on with a new partner. First you need that love to disappear and to heal from the betrayal. The healing will happen with time. As in your post in the New Beginnings thread, there will be good days, and gradually, the good will outnumber the bad until you reach indifference.
What boundaries or conversations could have been had, and how do I protect myself moving forward?
The best thing you can do is stand up for yourself. Don't sacrifice your happiness to save the marriage. Demand to be treated with respect, courtesy, and dignity, and believe that you deserve that.
Consult with a few divorce lawyers to understand what your life could look like after divorce. Get a clear picture of your financial situation, and make a plan for how you can afford to live alone. If you need to find work, start on that as soon as you can, or if you need to go back to school, get started on that.
Why do some partners keep what they truly want hidden until they end up destroying the relationship?
They don't. He is lying to himself and you about his degree of unhappiness with the marriage because he wants to justify his crappy behavior. I bet he wasn't "so unhappy" until the other woman entered the picture. Soon after DDay, WX spouted off about all the ways I'd wronged him over the years, but as he gradually pulled his head out of his a**, he admitted that our marriage was actually pretty good and that I'd generally treated him very well. And then when I told him I wanted to divorce, suddenly I was the best thing that ever happened to him and he couldn't bear to live without him.
People make up all kinds of internal narratives to avoid feel bad about themselves. Your WS is no exception. Don't accept the blame he casts on the marriage or on you.
[This message edited by NoThanksForTheMemories at 10:22 PM, Thursday, August 13th]