I felt the same hurt as you that my H was different with OW than he was with me, even after the MLC ended--he even composed 2 piano pieces for her! However, you have to remember that MLCers are essentially having a "redo" of their teenage years, making extravagant gestures and statements that they really wouldn't feel comfortable with when they're not running purely on their emotions. Research has shown that the teenaged brain is wired differently, and so is the brain of a depressed person.
So, just keep remembering that the OW he wrote to was only ever a fantasy, the soulmate he felt that he wanted at that point, but he never knew her in the way he knows you because he never really saw beyond his own needs. My H admitted, near the end of his OW obsession, that on some level he'd picked a person who lived in another city, etc, because he probably would have become bored with her if they were constantly together.
MLCers do spend like teenagers as well, and mine spent the most as he was in withdrawal from the OW. Keep an eye on his spending, if it continues as it has been, to make sure you're not going to be in financial difficulties down the road.
I'm glad you've continued with the Post sessions, and that your H is willing to help other couples--this is huge!
Meanwhile, though, do everything you can to keep your hurt and anger from spilling into your interactions with H. Do whatever you can to self-soothe and detoxify yourself from those negative emotions. And yes, detachment is required for Piecing, since your H has a lot of processing to do still and will not be available to be a real companion to you for some time yet.
Still, when you think about how far you still have to go--remember how far you've already come!
