Adult ADD ends 21 yr marriage with non ADD-trying to understand all the connections-please help!!

My 21 yr marriage with my add husband is ending in divorce and involves some very weird behavior and components.  I am just trying to understand it all and put it all together, because frankly no matter how much I do research, think logically for months, or ask for advice, I am still completely in shock, and feel like the man I knew no longer exists.  I just wanted to ask if anyone thinks this behavior could be ADD related or just another guy in mid-life crises.

Ever since we were first married I knew something was wrong with my husband, but finally put it all together one day after learning about adult add.  He had been diagnosed as a child and later told it would go away after puberty, and so nothing else was done.  After reading I realized all his behavior definitely indicated his add was still with him and affecting him now as an adult in different ways.  We both learned much, I felt like a bit of an expert after a while and he learned coping mechanisms that helped him and also what jobs he would be better at and so on.  We talked alot and really felt like best friends and sould mates and that we both understood each other very well.  And after many trials, tribulations, 2 kids (one with adhd), and me stuck with fibromyalgia after a hysterectomy, still together, coping, loving, trusting, I had no reason to think it would ever change, but I was so wrong.

I had no indications, no signs, no warnings, nothing, just it started with 3 emails saying he was unhappy, then he didn't know what would happen, separation or divorce, or what, then the final was he wasn't in love with me anymore, but he still loved me.  First that was strange enough after 21 yrs and he tells me all this by email????  Granted he was working offshore in Vietnam, but recently started work in a new dream job with several promotions up and also an increase in salary by alot, something he has worked for for almost 20 yrs and finally now we could live without money problems and he would enjoy his job, it all sounded ideal, a miracle, answer to prayers esp. when this happened right before we lost our house to foreclosure, it was amazing timing, we got our house back, did many repairs, and except for my illness, seemed great.  Little did I know, this "miracle" would lead to the the worst time in my entire life and so much pain, despair, and misery I could never imagine especially not from my sweet, kind, patient, moral, generous, laid-back, submissive, respectful, loving, devoted, romantic, and trust-worthy husband....and then all of a sudden found out he had been cheating for 4 months before saying anything to me, and cheating with a 22 yr old Vietnamese girl (we have a daughter who is 21), and to top it off he also had a rent house in Vietnam all set up as well.  When he came back "home" to the US that first time, and finally confronted me, I had no idea who this was, this was not my husband, I did not know if he had a mental illness, mid-life crises, being manipulated by this girl totally, or even possessed,  he was rude, arrogant, mean, cruel, and seem to have no feelings for what he had done and what it was doing to me (I was so sick, mentally, emotionally, and even physically I had to even go to the hospital where they upgraded my acute depression to post-traumatic stress).  His reasoning was crazy, made no sense, he couldn't even manage to say he was sorry...and it got worse.

Every time he went back to work and then had his 28 day leave, he would only come and visit for like 4-6 days, and his behavior was worse, erratic, and yet he still seemed to make him self at home and act like nothing had happened while telling me constantly of his experiences and relationship with pictures and all like we were still best friends and like he didn't even think about how it was hurting me.  Months went by, things still stranger, he was completely immersed in the Vietnam experience, the people, culture, food, language, everything, was entranced by it and seemed overly enthusiastic and obssessed with it, and he had a new girlfriend now, 26 yr old Vietnamese college student, and also a bunch of all female friends.  I found out sneakily how to send a message to this girl, because he indicated that noone knew that he was even married, and no matter how I tried to get angry I just felt too sad, hurt, and betrayed, and tried to forgive, keep loving and try to be there as a friend while logically trying to figure out and fix this problem....I did find out he had lied to this girl and everyone, and weirder, he not only said he was a single man, divorced 5 yrs with a son, he failed to even mention he had a daughter...and that is why I say it only get more bizarre...I did message that girl and set her straight and said he was not single, and had been married for 21 yrs and still was, but he also had a 21 yr old daughter and a new granddaughter!!!!  From what I heard later, she was really messed up but I felt just fine...until several days later he calls not mad, but happy, thanking me because he said he could now set the record straight...and even admitting it was his mess up, still not explaining why in the world he would just delete his daughter from his memory....and as I write this, It just keeps getting worse and I still demand this is not my husband, and the kids don't even know him either and of course my daughter is so hurt...and he is digging himself in deeper, now I don't see how he could ever come back, he even said he was happy now, with a "nice" house and lots of friends...(and of course the "other" benefits of dating really young girls with no consequences and basically having no responsibilities, but at the same time believe it or not he has been paying all the bills-mine and his new ones for these 2 different lives-or double life as I refer to it, and doing it with no real complaints because I am disabled, with no money and no job of my own.)  I developed fibromyalgia right at the beginning of grad school after getting my bachelor's degree, and then it manifested at it's worse the next year after my hysterectomy, which apparently happens commonly.  So, I never got to use my degree or reap the rewards.  But, he never complaned once and never blamed me, or so I thought till now.  He says he hasn't felt the same for like 5 yrs, and I had many chances, but will not explain what he means,  and although I remind him that he is talking about after I got sick, he either doesn't believe in the illness or me, and now acts like it doesn't matter whether I could help it or not, it happened and there is nothing I can do to change anything....he continues to display erratic behavior, and sounds even confused at times, esp. when my scientific mind starts asking him pertinent questions, and then as usual he gets all flutstered and says things like I don't know or What do you want me to say, and he still treats my daughter like she doesn't exist, he made a statement last time, he only came "home" meaning my house, to see his son...and it hurt because he still says I am his best friend and he does love me, but the worse thing is once again he spoke and forgot his daughter!!!!!  And when I pointed it out, he said I didn't say that or mean that...I have a therapist, and even she is confused and it's all too weird and bizarre that it almost seems like he's gone temporarily insane, but how and why and now, we have no clue...I know now from my research, that a few things I am sure of, he probably is in some kind of mid-life crisis, and he made some major health changes that improved the way he looked and made him feel much better, and also got his high-blood pressue and Type2 Diabetes (he had only been recently diagnosed with about 6 months prior), so along with a better self image, he might of also naturally helped his hormone levels, as well which would make him feel younger and revitalized, and maybe he could of felt that these feelings were "love" related instead of physical changes based on better health...he did claim the first girlfriend he had cried about and said it was love at first sight (saying all this to me of course, like I was a big zero and our long years meant nothing)!!!  I even told him a week ago he was here, that he was displaying either sociopathic behavior or was possessed!!!! And I meant it!!!!  He just laughed as usual and walked off whistling (his usual denial behavior or way of not dealing with stuff, acting like it doesn't matter)!!!  Well , sorry this was so long, I had to write enough in hopes that someone out there will make some sense of his behavior, I have tried, my family has, my therapist, my doctors, and even a week in a mental health facility with no help, no answers, and no hope.  At this point, although I did by best and just simply loved him, prayed for him and was just kind and friendly didn't help one bit, last time he treated me worse he was even more different but wouldn't tell me why.  Now I can probably only hope for his sanity and the kids to have their real dad back, as for me I getting tired of trying everything...Please help, I am 52, not wanting anyone else, never again, and I feel I have no future...I would still like to know the answers because I just can't let it go without some answers, nothing makes sense.  Anyway, I am the non add spouse, and although I have my own issues like OCD, if you can't tell, lol!!!