Every long relationship, at some point, runs into a wall the people inside it cannot see. That is when the friend who tells you the truth becomes the most valuable person in your life, and also the most dangerous, depending on whether you have one and whether you are willing to listen.
Most friends, when you start describing trouble in your partnership, default to a script. The script has two main settings. The first is solidarity: they hate the partner with you, validate every grievance, and reinforce whatever story you're currently telling. The second is neutrality: they listen, they nod, they say "that sounds hard," they refuse to weigh in. Both scripts feel like friendship in the moment. Neither is friendship. The first one keeps you stuck in a partial story that flatters you. The second one leaves you alone with a partial story that flatters you. The friend who tells you the truth refuses both scripts, and that refusal is rare enough that you might have only one or two such people in your entire life.
What they do is different. They listen first, long enough to actually understand what's happening, not just long enough to know whose side to take. They ask questions that don't fit the story you came in with — questions like, what did you do right before that, or what does he say happens, or have you considered that the thing you're describing as her flaw might be how she experiences your flaw. They tell you, when the moment calls for it, the thing nobody in your inner ring is willing to say: that you are the one who is being unreasonable here, or that the relationship is over and you are the last to know, or that you are about to do something you cannot undo for reasons you have not examined.
This is harder than it sounds, on both sides. To be the friend who tells the truth, you have to be willing to be wrong, to be misunderstood, to be temporarily disliked, and to absorb the cost of saying the thing nobody else will say. To receive that kind of friendship, you have to be willing to not need agreement, to let someone you love describe a version of you that doesn't match your self-image, and to sit with the possibility that they might be right. Most pairs of friends fail one or both of these tests, which is why the script-version of friendship is so much more common than the real version.
The first law here is humility, and it cuts both ways. The friend who tells you the truth has to do it humbly, not as a verdict from on high but as one limited perspective offered carefully. The friend who receives it has to receive it humbly, not collapsing into shame and not bristling into defense, but actually considering whether the read is accurate. When both ends of that exchange go well — which is rare and beautiful — the friendship becomes one of the most useful structures a person can have inside a long romantic life.
The cost of not having such a friend, or of having one and ignoring them, is years. People stay in marriages for a decade past the point where everyone outside the marriage could see it was over, because nobody inside their circle was willing to say so. People leave relationships in a week that, with a better friend, they might have saved, because the only voice in their ear was the one telling them they deserved better, full stop, with no examination of what they were actually bringing to the table. The friend who tells you the truth shortens these arcs by years.
There is also a question of which truth. The friend who tells you the truth is not the friend who tells you all of their opinions about your relationship. They are the friend who tells you the load-bearing truth — the one thing, the central thing, the thing you cannot see and need to. A truth-teller who comments on everything becomes background noise, and you stop hearing them. A truth-teller who saves their major interventions for the moments that actually matter retains the ability to land them. This is a discipline, not a personality trait, and the friends who have it are people who have learned, usually through painful experience, that the bandwidth of unsolicited truth is much smaller than it feels from the inside.
The work, if you do not currently have such a friend, is partly to find one and partly to become one. They are usually older than you, or have been through a divorce, or have done significant therapeutic work themselves. They have the calm of someone who is no longer performing perfection. You earn their truthfulness by being able to take it, which means working on your defensiveness for years before you'll be trusted with the hard sentences. You become one for someone else by doing the same work in reverse, and by being willing, when the moment comes, to lose the friendship in service of saying the thing it needed to hear.