Do narcissists ever get better in relationship, or does the relationship continue to spiral downward?

Narcissism is a Personality 

If this man is a narcissist, he has been his entire life. I do not think this is the primary purpose of your question, but I believe it is necessary not to allow his behaviors to be chalked up to a mental health disorder. Narcissism occurs in childhood from an environment where someone's sense of attachment or belief in oneself and the world around them is a sure way and then builds up behaviors to live in that world of detachment, and winner takes all.

A narcissist is actually a fragile human who overly relies on the ego to participate in life. This ego only cares about itself, and its betterment. The rule goes, the weaker or more scared the child, the stronger and bolder the ego has to defend it. Your husband would have had these traits your entire marriage; thus, I question any authentic happiness as defined by being together in emotionally intimate ways. 

Adultery would fall in line with a weak man. A weak man requires validation and fears missing out, fears being overlooked, and is absolutely devastated if rejected. Adultery is a way for the ego to feel powerful and worthwhile. The ego says to take and accumulate, and then you will be somebody. If he is a narcissist, your husband's adultery is a symptom of his inability to sit in the stability of a committed relationship. As a child acts out and seeks to devise more from an unstable environment, so did he. 

You may not be able to reconcile mostly because he would have to take responsibility for his actions and admit to being weaker than he realized. A narcissist won't do that. A few things will destroy a marriage almost guaranteed, and one of these things is a lack of accountability. We require a partner who will carry his part of the responsibility because it validates what we see in them. Without validation through your husband's accountability, you will form resentment. Resentment and contempt are cancer to any marriage.

If your husband messed up, confessed his sins, and gave insight into the nature of the wrongs he did, you may feel a different way about going forward. However, you know him and know that he will do it again once this pain passes and given another chance. 

Why?

Why, against all logic, would someone hurt someone and destroy a good thing? Because they are not driven by a rational mind but by the appearance of something they have never actually had, a genuine relationship. The ego says, "this next one will be better." This next thing will solve it. The ego has to do something; it cannot just sit and tolerate the pain of not having what it is entitled to.

As I said, the symptoms of NPD are that of a wounded child who has learned enough to overcompensate as an adult. This person lacks empathy because the situation is all about him. His cheating was probably twisted to how he was wronged somehow or the victim of some circumstance. Do not fall for that sort of speech if it is occurring. It is the ego self-preserving, and sometimes people have to go through rejection to realize they cannot get away with these behaviors and that they are not the person to be overlooked and assumed to be weakminded.