Any good psychotherapist will tell you that there are pretty much no rules for how a person should behave in a relationship. Successful partnerships can take many forms, and a person's cultural background, childhood, and past relationships all play into their actions and attachment style.

There is one piece of guidance that is non-negotiable for any couple, though: you need to overcommunicate.

This doesn't mean say everything that comes to mind. But therapists have found that couples who clearly express their appreciation and empathy for one another have longer lasting unions.

Amy Morin, a psychotherapist and author of "13 Things Mentally Strong Couples Don't Do," told CNBC Make It she has identified specific phrases that those in healthy partnerships tend to use.

"If you and your partner regularly use these phrases, it's a sign that you're already a mentally strong couple," she says. "And if you don't yet, you can start implementing them and find that you'll grow stronger both individually and as a unit."

Here are six phrases successful couples use, according to relationships therapists.

1. 'I'm going to tell you something that may be upsetting to hear'

Withholding potentially hurtful information from your partner isn't healthy. Even if you believe it will make them uncomfortable, it's best to address any concerns you have. Just be sure to do it in a tactful way.

Pre-empting news like "I felt uncomfortable when you told your mother about our private business," with a phrases noting that this information will be upsetting can demonstrate empathy.

"Acknowledging your mistakes and being honest about your needs can help you grow stronger together," Morin says.

2. 'I need your support right now'

To find out what makes love last, clinical psychologists and researchers John and Julie Gottman interviewed more than 3,000 couples and followed some for as long as 20 years. There is no formula for solving conflict, but some conversations are more fruitful than others.

"Your mission is to allow yourself to be vulnerable — to turn attack and defend into self-disclosure and openness," they wrote for CNBC Make It.

Saying "Can I have a hug?" or "I need your support right now" can signal to your partner that you are being flooded with emotions and need to take a moment.

3. 'It's understandable you feel that way'

Just because you might not feel the same way as your partner doesn't mean you can't validate their emotions. Saying, "It's understandable you feel that way," can be reassuring and show you have empathy, Morin says.

4. 'I never thought of things that way'

There will be times when you want to meet your partner halfway or at least validate what they are saying. The Gottmans call these "getting to yes" phrases.

By saying "Let's compromise here," or "I think your point of view makes sense," you're showing that you're listening to your partner, not just trying to establish your point.

5. 'I'm sorry for the part I've played in this'

Owning up to what role you played in a conflict is essential for mutual growth, Morin says.

"When you take responsibility for your share, you increase the chances that your partner will accept responsibility for theirs, too," she says. "Then you can both put your energy into developing a solution, rather than getting stuck pointing fingers and arguing about who caused the problem."

6. 'Let's find a solution'

Successful couples solve problems together. Even if the challenge is outside the relationship, collaborating to find a solution can help strengthen your bond.

"While some problems are ultimately in your partner's hands, like an issue they're having with their boss, offering to work together shows that you're invested in helping them make the best decision for themselves," Morin says.

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