All couples have ups and down. My marriage certainly had a big down 30 years ago, alternating between fighting and indifference—The latter felt worse.
Barbara and I did therapy but it didn’t help enough.
I then suggested that we, separately, privately, write what we liked and didn't like about aspects of our marriage: sex, communication, children, money, career, and recreation. For each, we wrote one behavior we would commit to changing and one behavior we wished our partner would change.
Then we showed each other what we wrote, agreed on what each of us would try to change, and that we'd meet in a week to rate our success.
During the week, when we saw our partner doing something good, we'd give a thumbs-up, something bad, a thumbs-down: No lectures, no recrimination. Just the thumbs-up and thumbs-down.
The next week, at a dinner out, we rated ourselves on our success in implementing the new behaviors and asked each other if s/he agreed with our self-ratings. The "meeting" ended with our agreeing on the behaviors that each of us would work on the next week. Key was that each of us proposed behavioral changes for ourselves, not for our partner. The partner's job was only to okay it or to suggest an alternative. We then just enjoyed dinner, intensity over for the evening.
We had those weekly dinner-out meetings for about two months, and that has remained key to keeping our marriage on a more solid footing. It's been 30 years since Barbara and I had that marriage summit. We've now been together for 41 years and hope to continue for a long time.