Hi AmyTX - I am sorry your h did not support you during your health issues. I am betting that if you think about it you may see that your h always had issues with aging, sickness, hospitals, etc.? I think many, many MLCers do. This is not to excuse what he did. But this fear of aging/sickness + bad coping skills = perfect MLC formula.
About the therapist, that's good you are talking to someone! Be aware that there are very few therapists who really understand MLC. Even though they say they do, probably they get midlife "transitions" (I am not where I want to be in my career, I hate the empty nest, etc.) but not midlife "crisis." That is a different story.
Here's my funny therapist story. I sought out someone who was pro marriage and who understood MLC. 7 months post BD, I go to talk to her. She is taking notes and nidding. I am telling her that at bomb drop my told me he wanted to live at home, get an apartment, sleep around on Saturday nights, he told me I could sleep around on Friday nights (!) and then the rest of the time we would play Leave it to Beaver. (We had a completely normal, monogamous marriage, nothing kinky! No farm animals, power tools, etc.) She looks up and asks me how long we've been swinging! (I do live in SoCal and that sort of thing does happen here so ...)
Point is, just because a therapist says she gets MLC, does not mean she really does. Most do not understand MLC nor the corresponding regression, confusion and depression. So don't be surprised if your therapist tells you to cut and run. But, you can still work on you with him/her.
About your h wanting to date you and other women? That is called cake eating. He wants to keep all his options open. However, we teach people how we are to be treated. When my h would come up with his "plans" (that ALWAYS only benefitted him) I would think: what if my kids were watching all this? How would I want them to see me acting as a role model? And remember: if you have maintained your dignity but lost your marriage you have not lost a single thing!
Stay active and take care of you and your daughter.
Me 41, H 47, M 15 yrs, S11, S13 BD 1: 11/4/14 we work on it; really I pretzel myself BD 2: 3/31/15 H goes down to "dorm room" 8/15: H back to MBR 10/15: H back in dorm room 1/18: H files, now divorced