You three make me laugh. Even in the sadness there is humor and compassion and a kind of sisterhood. I remember some days journalling, small silent tears running down my face, and DV or Yorkie would respond with something funny (king douche bag of douche bag land comes to mind) and I would find myself unable to stop giggling or with something simple like "your H is an idiot" and even though it doesn't seem like much, those words meant the world to me.
Hope - your therapist is correct. My M was dead. It was dead because it was broken and it had been broken a long time. The need to escape was probably building up inside my H for well over a year. That need was probably building up inside me too. If we had known earlier, it could have fixed it, but instead we pushed it down, letting it seethe and grow inside us. Once my H made the decision to go, he was always going to stay gone, because no matter how much he wants to come back, he is still afraid that things haven't changed, we will be back to where we were, but this time he won't have the courage to leave again. Clarity comes after the fact, never during.
That does not mean there is no hope for us. There is always hope. But in the meantime you need to live as full a life as you can. Fill it with things that make you happy. If you don't know what those things are, then try everything until you do.
The cake eating thing is a difficult one. It is only cake eating if you have some expectation that your 'niceness' will somehow bring them back. That your kindness will be reciprocated. It won't. The only thing that will make them start being kind and compassionate back is if you set boundaries, and even then it isn't guaranteed. Be nice for you. Be kind and compassionate because these are good qualities to have. Lead by example, but don't expect to be followed.