What would be the secret of a relationship that lasts and helps us to grow?

"What is the secret of a 'perfect' relationship?"

The secret lies in the word "mirror" -- what you see in your partner is what you are perceiving your partner to be and therefore mirroring back those feelings.
Intimacy in relationships is the first real step towards growing together. In intimacy arises expectations and with expectations arises also resentments because of how we expect things 'should be' in the individual mind. This stage can't be helped since it is in the process of 'growing' up emotionally. It is at this stage that we learn 'forgiveness' which is the ability to embrace your partner as he or she is according to their enneagram type.
Forgiveness, which is the generous open willingness to embrace whatever IS, is the secret key to a mature spiritual relationship. Forgiveness is not just for the unconscious expectations but towards the deeper subtle resentments that underlie communication. A common resentment among the left-brain mentality is "he/she doesn't make sense when she/he is upset." The right-brain mentality harbors the resentment, "he/she doesn't listen." or "he/she doesn't understand me."
Communication is a major factor in a relationship and unless it is motivated by and through genuine love and respect, it is bound to create unconscious deep-seated resentments.
Two people, no matter how much in love they are and embrace intimacy, are bound to differ in their views of life (as the enneagram types suggest such diversity) and yet the truth is that no one is right or wrong. The key point is -- are you willing to forgive? The ACIM Course suggests, "Would you rather be right or happy?"
Step inside your partners feelings and generously look from within their viewpoint, in other words, mirror how they see. No one is right or wrong...only LOVE matters.
The greatest human fear is rejection and this abounds in relationships and creates defensive attitudes that gradually turn to deep resentment and finally even hate. Many relationships are imprisoned by love/hate relationship. It is through this fear of rejection that resentments are built and finally lead to the death of love. Resentments create the feeling of separation and even after being married to someone for twenty years or so one might feel, "I have married a stranger."
All forgiveness (embracing, allowing, accepting, surrendering, seeing their illusion etc) of resentments is the only needed requirement. Therefore awareness of any inner resentment, no matter how seemingly slight, is the first step towards a great relationship followed by forgiveness of such. Awareness not only of the words uttered in communication with each other but also the personal projection that ensues automatically from the given person. For example, if we suspect a certain attitude and feeling behind the words then we are carrying that within us and blaming it on the other. This projection of seeing the other as offensive is really one's own belief system (mirroring). We see only what we are ourselves. To see love rather than fear. Our only choice in communication is choosing love over fear every time remembering always that there is no right or wrong but love or fear. Love will grow or wither according to the view we have of it -- are we seeing Light or darkness?