... he has just decided that he's tired and feeling "beaten down" by me. Says that I made him feel really bad about himself for a few years now (funny because he didn't say anything about that until 2 weeks ago). ... I feel like aliens invaded my once loving and loyal husbands body. I have read about the "fog" and it sounds like what my husband is experiencing.
First of all, I am very sorry to hear you are in this situation. You are not alone; there are dozens of us on this site who can relate to what you are going through. It is an awful experience which you don't deserve, whatever your own issues in the marriage.
I can relate to the "alien invasion" -- that's a great analogy! The sudden change in personality, the blame deflection ("it's all your fault"), the self-centeredness ("you didn't make me feel special so I am not going to work on our marriage, I am going to step out"), and the utter lack of responsibility ("I am going to leave you with the kids"). Sounds like the fog has indeed claimed another victim.
I don't know what your best course of action is over the next week or two, but the good people on this site will give you some useful coaching.
My only suggestion at this point is for you to try to maintain your dignity as best you can -- while you are 50% responsible for the marriage, he is 100% responsible for walking out. You are a loyal wife and mother. You deserve better treatment than this. It is difficult, but keep telling yourself this. Anxiety is normal in this situation, but do not surrender to any kind of paralysis or depression: keep as active as possible, focus on your kids and other positive things.
My best wishes in sorting through this difficult situation.