How to rebuild TRUST when he won't stop LYING?

I am new to the forum. Like so many others on here, I am perilously close to divorce. I REALLY don't want that to happen, but I just don't know what to do. My (fairly story-independent) questions are at the end, so you can skip the background story or read it- whatever you prefer :)

I have been with my husband (recently diagnosed with ADD) for 17 years- three of those were long-distance while we were in college. After seventeen years, five cities, building a small business together, and plenty of trials and tribulations, my trust in him was truly monumental. I couldn't understand the scale or value of it until it was gone.

Eight months ago a friend emailed me to tell me that he'd found texts on his girlfriend's phone indicating that our partners were having an affair. I was stunned because our marriage was solid- affectionate and happy and good! I confronted my husband, he admitted that he did have "feelings" for this younger, prettier woman (also a friend of mine), which absolutely crushed me, but that he had never transgressed beyond some irresponsible texting the night before and a couple of secret lunches. I believed him- I didn't think he would ever lie to me directly. I actually thought my husband was so ethical that it would make him physically ill to cheat on me or lie to me. (Sigh. I know.) I thought that he'd only managed to deceive me because it had just started days before, and had only been "lies of omission."

After much tears and drama and reconciliation, we were two weeks recovered when I found out that he had in fact told me a bold-faced lie. That there HAD been physical transgressions, that one had happened over a year before, and that he had been keeping it from me and hanging out with the three of us for a year without ever batting an eye. I freaked out in a BIG way. Who was this pod-person? He swore that he'd so thoroughly put it out of his mind that he never thought about it during that year, that it had just started up again, and he'd only lied when confronted to protect my feelings. Again, I believed him!

Now the ADHD part. The stress of this whole situation made my husband's formerly mild ADHD explode- but neither of us knew what was happening. All I knew was that I needed my husband to be more sensitive and thoughtful, and instead he was way, way less. I couldn't understand why he was acting this way- I'd learned not to take his ADD (which I thought was just "spacing out when I was talking") personally years ago. I didn't understand why he'd say he was going for a five-minute walk and then come home from the grocery store and hour and a half later. He'd say maddeningly contradictory things in arguments that went in circles and veered off on tangents constantly. He told EVERYONE about his affair. He told our friends intimate details about our new sexual problems. He wrote a song about how hurt he was that the other woman moved on so fast and debuted it at a party in front of all of our friends. He broke promises, denied having made them, and almost never expressed remorse after doing these things. He started getting extremely angry and raging, often frightening me, when I would get upset because of something he'd done, no matter which way I tried to talk to him about it. He seemed to have zero empathy or compassion. He'd read the same affair recovery books that I had, but just wouldn't do any of the emotional things they said were essential for rebuilding intimacy and trust. On the contrary, he did such hurtful things with such regularity that I thought he must be doing them deliberately, and I couldn't understand why he would hate me so much to do that to me.

I finally found a marriage counselor who was highly educated, available, and on our insurance- but our visits to her actually seemed to make things worse. She clearly didn't believe me that any of these things were happening, and I got the distinct impression from her that she felt my H's affair and our subsequent problems were my fault. After a couple of months of this I just couldn't take it any more and told her it wasn't working out. She wrote us a letter where she urged me to seek individual therapy for my "anger problem". Of course, this was before we knew anything about ADHD.

One day we had a fight because my H told me he would do something, and after waiting an hour I found him on YouTube. He bought me a book on Adult ADHD, hoping that I'd get off his back about his tendency to be distracted. Instead, it explained SO MUCH MORE than either of us had ever known about ADHD. I was simultaneously ashamed and sad that I hadn't known more about it sooner (in my defense, he was really high-functioning before the affair- no money, employment, anger problems, etc), and so relieved that his behavior wasn't stemming from lovelessness or malice. I had such high hopes that we'd find a doctor, find a therapist, find a good drug regimen, and finally be able to begin rebuilding trust.

All of those things eventually happened- except for the trust. The sense of remorse, compassion, and safety that I'd been desperately craving for months never manifested. Instead my H started acting like it had been four freakin' months- I needed to "let it go" already! He started complaining that he was angry and unsympathetic because I was smothering him and was too sensitive and/or parental (because I wanted accountability and transparency, and still cried too much). He literally stormed out of every important conversation- often after becoming violent. Then, after months of truly believing that he would never lie to me again, I caught him lying, and this time there was absolutely no way it was for my benefit- just his. He chose to break an important promise- for hours- and then lied to my face about it over and over for days. I watched him do it and just stopped caring if he left me for another woman.

Eventually I told him that I knew he was lying. He was indignant, and then when he realized I had proof, made an unrepentant apology, said I was wrong to force him to make that promise anyway, and tried to sweep the whole thing under the rug. I felt emotionally paralyzed, but we were planning to move cross-country, and I suppose I still hoped that things would change in a new city. I told him to go there alone to house hunt, and not hurry back. He went, and was as un-smothered and un-parented as one could be, but the explosive anger and tantrums continued any time I said something he didn't like- including those times I was honest about my lack of trust. He would bellow at me on the phone that one teensy lie shouldn't undo all the other times he DIDN'T lie to me. If I argued he would hang up on me. I was so tired of the drama that I stopped speaking to him for a couple of weeks. When he came home he was much nicer- for a couple of months. As soon as I started to believe he really had changed, he started raging and abusing again. Two weeks ago he really scared me and I took my pet and a few things and left, driving 14 hours to my parents' house. He called me and told me to come home. When I refused, he said he wants a separation. I said I want a divorce because I don't trust him not to hurt me during a separation. He was furious- he didn't see why I shouldn't trust him- that last big lie was months ago- I'm just as deceitful as he is- there's been plenty of times he didn't lie to me- and- (this one got to me)- I'm the one who's bailing on our marriage, because all he wants is a healing separation so he can "work on himself".

I was dubious. I thought he would spend a lot of time playing video games and drinking with his buddies, but I didn't believe that my discomfort-avoiding H would actually confront his demons. But I decided to try this one last thing- provided we work out a written separation agreement so I would feel somewhat protected. Well- that's a lot easier said than done! Despite my following all of the non-violent communication rules, he still screams at me and hangs up any time I express a different opinion or refuse to give him his way. He's been dragging things out, indeed- playing video games, and who knows what else. A couple of days ago he lied to me AGAIN, and I realized that the agreement is pointless, because I probably won't know if he breaks it- and I fully expect that he will. I don't believe a word out of his mouth anymore- how can i be married to someone and feel like that?

So I'm at a loss now. To anyone who's made it through my long sob story- I have several questions.

1- I know many people on this forum are dealing with infidelity, incessant lying, or lack of empathy. Has anyone successfully dealt with all of these?

2- Has anyone found a separation to be helpful with ADHD-related marital problems?

3- Is it possible that his drug regimen is inadequate and that a different med or dose would allow more empathy?

4- Is it possible that co-morbid depression could be causing the anger and lying- even though he appears happy sometimes (such as when we go out with his friends, etc)?

5- Is there any hope with this, or do I need to accept that he will never provide the emotional reliability that I need as a result of his affair?

Thank you so much to anyone who has any advice!