He says can't begin to be a faithful person until he fixes what is wrong within him and only a therapist can help with that. But he hasn't started therapy or even started the process.
I agree with him completely, he absolutely does need to fix himself and his past issues first. Kudo's on that realization.
However that's like saying "I should really wash the dishes" and yet still sitting on the couch watching TV. Knowing what needs to be done, and actually doing it, are two different things. He knows "what" to do. But something is stopping him from doing it. And part of that, honestly, is most likely whatever he needs to address in therapy anyway.
16 years is awfully long time to still be floundering over getting therapy. Or going back to it. I'm going to take a risk and make a prediction... if he's had sixteen years to fix this and still hasn't taken steps to do so... he's never going to. On his own volition that is.
My wife says that no one makes changes in their lives until forced to. I find that to be true. Right now he's not doing the work, and he's not caring enough about you and what you need in order to make those changes in his life. So you get nothing of what you need from him, but it sounds like he's still getting everything he needs from you. Any wonder why he's not making the effort?
I can't tell you what to do, but I can make a suggestion. I would suggest having a "come to Jesus" type of moment and tell him that this is what you need from him, that he needs to go get help and make progress. What he does or doesn't know, what he feels or doesn't feel, what he cares about or doesn't... has no bearing on what YOU need from him. If he wants you in his life, then he needs to make an effort to provide what YOU need.
When it came time to have an affair, I'm sure he faced challenges. He had to fool you and lie about where he was and what he was doing. He probably had to spend money for things and hide that from you. He had to find time, and locations, to meet, and then come up with excuses and lies to tell you, and had to keep those ruses up. He had to find time to write emails and texts and phone calls, time to go out, be gone for a while, to find safe places to meet or talk or hook up. That's a hell of a lot of effort, time, energy, cost, figuring out, overcoming, adapting and changing to get what he really wanted, and guess what? He got it done!
So where is that same level of effort now? Where is that, "I'll over the challenges and odds needed to make this happen" when it is something YOU need? Doesn't HE want this marriage to work now? Doesn't HE want to be a better person? There where is that "get it done no matter what" attitude?
Please protect yourself. From this end of the internet, it sounds like excuses and justifications coming from him, which sounds a lot like where most WS's are at during the affair. Same patterns of thinking and acting anyway.
THis isn't about punishment for him. It's about consequences. It's about working hard at a relationship. It's about doing the right thing and being a self-respecting person. He needs to stop making excuses and shit or get off the pot.
Give him a goal, and a deadline. If not, also give him consequences.