I don't know if this will help, but some of what you say rings a bell in my relationship. Also, some of what the other responders have posted was pertinent to us as well.
My wife, I guess you would refer to her as FWS, did not know why she had the affair. Hers was short, just a couple of weeks to a month, she doesn't even remember.
Alcohol, Antidepressants, Marijuana, all mixed up together contributed yes, but why? She went crazy over "why", long before I knew about it. After I knew, and I kept asking "Why?" and demanding to know the answer, she went even crazier. Imagine doing something that everyone in your FOO did, something that you had sworn to yourself that you would never do, then doing it, and not knowing why you did it.
It took her a long time to start figuring that all out. Years actually. When she started really talking she ended up suicidal and was eventually hospitalized for several days.
Your husband is angry, has every right to be, he doesn't know why and can't understand if he doesn't, and if you don't know then how can he know.
But, what he doesn't realize, is that if you could carry on an affair like this for this long, and do the things that you did, you have a lot more work to do than he could possibly imagine.
Yes, get the book, and a couple of others, and STUDY THEM. You will find things in there that ring some bells. Write them down, but remember that they are only leads.
Figure out what you got, emotionally, psychologically, and physically from the relationship. Figure out why you needed that. Why would you need and want such a "secret" and to put one over on people that you know intimately. Was it a feeling of power? Was it a feeling of security? What was it?
Someone's brilliant IC brought up "disconnected". This is very important. You need to read and understand what it really means, how it manifests in our lives, what causes it in others, and figure out what it's origins are in your life. Because, you simply cannot do what you did and not be "disconnected" from others when you are doing it. People who feel "connected" don't do this. Why don't you feel that connectedness?
Finally, he is absolutely right, if you don't know why, and don't put the work into figuring that out, it will probably end in divorce, sooner or later. Sooner if he just can't take the lack of understanding any longer and decides to move on with his life, in a safer situation. Later when you, not understanding what you did or why you did it fall back into the pattern that you don't recognize and repeat it again.
What did my FWS learn: Lessons from FOO
Daddy cheats, Mommy cheats, Everyone drinks, Everyone does drugs, Everyone cheats, you can't trust anybody, those who love you betray you sooner or later, rapists get away with raping you when you are sixteen, when you go to your parents you get told to "keep your legs crossed" and "it wasn't rape unless you had an iron bar slammed into your head", your husband will leave you, your father will leave you, your mother will leave you, after your parents leave you it is your fault when your uncle gives you drugs and you wake up on a couch in your underwear, Marijuana helps you forget all this shit, Alcohol helps you forget all this shit, Drink/Smoke more if it is becoming overwhelming, sex is bad, sex is dirty, sex is to be used to control people, women who have sex are bad, don't tell anyone about any of the bad stuff, if you tell anyone they will leave you or blame you, never talk about it to anyone, don't talk to counselors either, guys won't like you if you don't have sex with them, sex should not be enjoyed, and on and on and on and on.
Which led, in her case, to being disconnected in her life, in our marriage, and fear that never ended. This in turn led to the affair, it was another futile and misguided attempt to "connect" and doing so in a way that is destructive.
You will need to read and study a lot, study yourself, your FOO, and your life, to understand what it all means. Only then can you let him know. But, you need to communicate with him NOW because if you don't, all he is going to know/believe is that it was something lacking that was not intrinsic to him, something that the other guy did not lack, that would lead you to do what you did and to do it for so long.
But, it is not him, it is all you, and it is not the other guy, it is all you. He needs to hear that over, and over, and over.