New here, what to do with husband who won't even try?

I've been reading a lot of the posts and comments here, I'm amazed by how much ADHD can take over a marriage and encouraged by seeing some folks who are working towards making it work.

My husband has ADHD, undiagnosed, but I am pretty sure (I work with students with disabilities) and on top of it he has sustained a minor brain injury in a motor vehicle accident. I have been trying, in vain, to have him get diagnosed (got a referral for a good neuropsychologist and we have the insurance to pay for it). He admits his brother has huge issues with ADHD, and I've tried to let him know I see the same issues in him as well. He will not admit it. I've tried specific conversations on the behavior that makes it difficult to have a full partnership, such as the fact that he does not remember things, no matter how many times I tell him. He will even repeat the same things over and over to me. (usually, his schedule or information he wants me to remember FOR him). He told me last week "Well, I would say I would do better but why bother when it's a lie and it's never going to happen"

I am so frustrated at this point. I have a teenage daughter from a prior marriage, but with him it's like having 2 children, and my daughter is much more self-sufficient. Every time he doesn't retain something I feel marginalized and I have chronic depression (for which I have been on medication and seeing a therapist myself) for years.  I don't know how to get him to take some kind of step, some kind of responsibility, short of serving divorce have those of you with spouses taking some responsibility for their ADHD managed to get them there, encourage them to take the step?