I'd second the congratulations for sticking with it and trying to resolve things.
You haven't gone into to much detail in your OP, but I was wondering if this might be a different way to look at it...
It seems he is/isn't doing something you want, you pick him up on it, then he tries for a bit to do/not do it, but ineviatably fails at some point. You then feel let down and not listened to and he feels he can't please you because he tried his best. And so the cycle continues.
What about trying to accept him as he is? I know I've made it sound overwhelmingly easy and, of course, its not. However, one of the things I have learnt very recently is to pick my battles and truly accept my man for what he is.
For example, I feel my man doesn't always take time to consider me and how I would feel about things. I used to get upset, cry and get angry that he couldn't see that what he did/didn't do was hurtful. And I used to think it was so bloody obvious I'd think that he must have done it on purpose.
I now accept that as a baseline he loves me and would do anything to make me happy. However, he doesn't actually KNOW what would make me happy. When I speak to my girlfriends they totally 'get' me and see why I feel like that, but he has no comprehension at all!
Every so often does something that he doesn't realise hurts me. When he does I either ignore it (if small) or try to say why it hurt me. He usually apologises immediately and then I try really hard not to keep sulking! It stops the cycle and one is left 'trying' to change themselves.
As he told me one time, 'don't you think you do things that drive me crazy all the time, but I hardly ever mention them, as I know you love me'. That was a real shocker to me, not because i think I'm perfect (!) but because I'd assume he would tell me if I annoyed him. I think men and women tend to be different in the way their minds work.
So I'd say why don't you try picking your battles, and agree to resolving them as quick as possible. Asking people to change when they have shown over 17 years that they can't, seems to me like banging your head against a brick wall!
Of course i don't know the extent of what is happening in your relationship. If you are talking about affairs or abuse or any other 'dealbreaker' then ignore me totally!
I hope that that might offer a different way of doing things. Even if you ignore my advice completely, I would most certainly state the obvious, you both have to change how you react to issues, or else you will end up doing exactly the same as you have for the last 17 years!
Good luck, I am sure that you can get through this if you both work at it.