Is the illness part of the reason why communication needs some work?
I ask because sometimes (often) caregivers put themselves aside for the sick person. It is very nobleand loving, but a relationship needs more. If a caregiver keeps in some things because they don't want to hurt the person they are caring for, and/or the patient keeps in some things because they don't want to be disrespectful or ungrateful, things will build up and start erupting randomly, sometimes in response to small negligible things.
I would like to make a suggestion, that you write to each other or something along those lines. I struggle with communication, the thinking before you speak is something people have been telling me to do most of my life. I am introverted, reclusive, fiercely independent, and I am on the autism spectrum so communication is hard. One thing I have found helpful is writing. If there is something I need to communicate to my partner and I don't want to mess it up, I write about it. I could take hours a day, a week or more. I start with my idea, and gradually add to it. Writing it in detail first gives me an opportunity to really explain what I mean and where I am at as a person, and it allows me to make sure I emphasise that I value the person I am with and the relationship. So I literally send emails or long texts to my partner who is sitting next to me in person. They get shocked at first, but get used to it with time.
Is this something you would be willing to try? It doesn't have to be written, it can be audio notes or videos or something along those lines.

Hey J101,

Well, you 2 should give yourselves a giant pat on the back for getting started.

May I suggest that, when you and your wife hit a bad moment, you immediately say to each other that you can talk with the counselor about the bad moment, too? That way, during the bad moment, you both know that you both want to do better but in that moment you don't know what to do to make it better, AND in that bad moment you still are committed to your relationship and you are saying out loud to each other that you are committed, even though you might be really angry/frustrated with each other.

My husband flat out refuses counseling. Very frustrating for me.

In our marriage, I am the sick one. I started counseling 3 years ago, in part, to learn to adjust to my new life of chronic pain and disability. It has helped me a lot. Some of it does take a long time. And sometimes you suddenly --poof ! -- understand something really clearly.

To get the most out of my counseling sessions, I have found it is really important for me 1) to write down stuff while I am talking with my counselor, and 2) read over my notes and really think about it, and 3) really try to act on it. It feels kind of silly sometimes, but I will read a thing and then do it, whether it seems like I need to then or not. Just kind of play-acting and practice for when I do need it.
I have an audio-processing deficit, so I always have needed to write stuff down if I want to remember it. Maybe you don't need to write notes, but I suggest you do the thinking step and the play-acting/practice step as much as possible. These steps have helped me get a lot out of my counseling sessions.

Oh, also, I always prepare for my sessions by mentally reviewing what worked and what didn't work since my last session. I write a list of what I want to talk about with her, too, so I don't forget and so that I have a goal for that session.

Hope some of this may help you guys...And good for you for working on it!!!