When it comes to rules or advice for long-term marriage, I often seem to hear words like "covenant" and "promise-keeping". These conceptions of keeping faith with another person are laudatory and fine -- and even necessary to keep the love going.
But "covenant" and "promise", excellent as they are in principle, are insufficient to give you the oomph, the power, to do the thing you both want so much to do. That is because if something comes along that you actually find you want more than the thing you "covenanted" to preserve, or "promised" to keep, the covenant and promise will go out of effect. Covenants and promises are susceptible to being superannuated in exactly one minute. "It Only Takes a Minute, Girl, To Fall in Love" (Tavares, 1975).
How else could you explain the thousands, even the millions of sincere people who made a covenant or exchanged a promise which they then broke? It's not that they wanted to be untrue. It's just that something presented itself that felt superior to the prior thing. So the covenant got broken.
This is why I major on romantic love as the key to a happy marriage. When romantic love is there, between two people, they don't need words of encouragement to stand by a prior agreement. They don't need one single pointer to a covenant. "(All I want to do is) Act Naturally" (Ringo/The Beatles, 1965).
The one thing you can do to renew a relationship that's gotten stale or even repellent (God forbid) is go back, in your heart, mind and memory, to the point when the relationship was fresh, fantastic, and enthralling. And there was almost definitely a time when it was -- when it was fresh, fantastic, and enthralling.
The magic was there. (Otherwise, why would you have come together in the first place?) Apply the incomparable insight of Meister Eckhart, by way of Gerald Heard: "If you can't find God, go back to where you lost Him."