For many pressures are contained and handled through displacement. Unhappiness is buried in the busy-ness of living, depleting lives of vitality, with recurring low level depression and other ills.
Sometimes these emerge in personal crises. For example - a partnership is threatened by various kinds of infidelity or by one partner undergoing what commonly is described as ‘mid-life crisis’. These are complex and very serious disturbed processes - where something causes the suppressed parts to break out and disrupt a person’s identity and sense of reality. Often these are to do with splits which occurred in earlier life through an individual’s natural processes being over-constrained or denied.
It can be difficult to understand when a partner reveals unknown aspects of themselves, gives strange accounts of past events completely at variance with your own - or seems to act completely out of character. This is often not helped by the public nature of what happens and interventions and comments of friends and acquaintances with biased, partial and ill-informed perceptions.
Even so, with the right professional understanding and support, the relationship is not necessarily beyond repair and recovery.
Do any of the above sound like things you need to address ?