It seems to me that all problems in relationships begin when we stop feeling each other. Over time, people relax, move away from the general wave to which both were tuned in at the very beginning. You seem to be together, but there is not that unity when you finished phrases for each other. Harsh and hurtful words are spoken more often, but apologies are spoken less often. You can already leave the house without kissing goodbye and come back later without warning about it. All these are such little things that are no longer important. However, it is on such little things that a happy life together is built.
Therefore, when in any quarrel you have a choice - to hurt a loved one or not, choose the latter. And it doesn't matter if it's a word or some kind of action, bite your tongue and go into silence. Problems need to be solved spoily, without negative emotions. After all, it is very easy to break even what has been built over the years.