OP - It must be really so difficult not having the sort of closure you need. I really sympathise with that and think it would have made me feel like you. However, I really think it would help you if you did some reading about affairs and their causes, but please be judicious about what you read, as there is so much nonsense peddled about infidelity.
First of all, no - affairs are very often not about an attempt to get things that are "missing" from the marriage. That is one of the biggest myths of all. In fact, people who have affairs are often the most over-benefitted ones in their primary relationships - they actually take far more than they give. An affair is yet another example of them taking something extra. It is no surprise whatsoever therefore to read that your exH was always selfish and immature.
Secondly, the affair was not about you at all, or even your marriage. Infidelity is after all a pretty extreme and unstraightforward way of expressing dissatisfaction with a person or the relationship. If your exH is now demonising you or your marriage, tell yourself calmly that he had an adult choice that he neglected. He could have told you at any time that he had difficulties with you as a person or with your marriage, but he didn't do that at all - instead, he deceived you and had an affair.
The behaviour you are seeing now is merely guilt transference. Instead of holding a mirror up to his own flaws as a person, he seeks to justify his behaviour by pretending that it was your behaviour that led him to do this.
Keep telling yourself that this is not real - and try to see his "defence" logically, always asking yourself how an emotionally intelligent adult would respond to problems in a marriage, feelings of low self esteem, the proposition of an affair and finally being discovered having had an affair. Do any of his responses fit? No, they don't. It's not how you would have reacted as an adult and so his take on this can be easily dissembled. His views and justifications are worthless.
In your shoes, the best way of looking at this is that you were always the better person. Unfortunately, you were married to someone who never really grew up emotionally. Not every man is like this though - and you will not choose a man like this again. You can hold your head up high that you behaved like an adult at all times.
The best way to treat him now is with an air of utter contempt. Don't ask him for reasons, just nurture yourself and any DCs you might have. Life will get better for you, but believe me, it will not get better for him. Emotionally retarded adults never really live happy lives.