Some thoughts from a Catholic priest Fr. John Bartunek, LC that may explain why, in MM’s case, the number “seven” reflects that hers was the deepest and most comprehensive form of demonic possession:
Not all psychological difficulties can be classified as mental illness. Some are linked to changeable patterns of behavior or basic human maturity issues. These can be remedied by healthy living, sense of community, human and spiritual formation, the discovery of a mission in life, and other fruitful activities. For example, sometimes mild but persistent depression or problems with anger management are really rooted in patterns of sin that have torn apart a person’s interior balance.
It also must be said, however, that mental illness is a reality. Mental illness goes deeper; it is a dysfunction or disorder rooted in the structure of the personality. Mental illness in these cases is not always caused by direct demonic activity. Many (probably most) times, it is caused by traumatic physical or psychological experiences, or genetic/physiological imbalances, or a combination of any of these factors. In these cases, sometimes medication can help a person lead an almost normal life. Other times, good psychological counseling or treatment (“good” means in harmony with the Christian understanding of the human person) can help a person lead an almost normal life. In some cases, however, the illness is so deep that even medication and sound treatment can only help contain the problem, they cannot completely solve it. This is a life-long cross for the person and their family. Nevertheless, in all cases, from mild to extreme, a healthy spiritual life (prayer, sacraments, Catholic fellowship) should be developed. Discovering God’s love, and learning to live in its’ light, is the only path to full healing for all of us.
We also have to reiterate that demonic activity is real – the Church teaches this clearly, and it is obvious in the Gospels. The devil normally works by stirring up temptations in the areas of the seven deadly sins (greed, lust, anger, sloth, pride, envy, gluttony). Sometimes, the devil and his buddies work in more supernatural ways. This can be through “obsession,” by which a demon will bother a person from outside, causing chronic physical, physiological, or psychological pain. This can also be done through “possession,” by which a demon takes temporary control of a person’s physical and bodily capacities from within (but not their soul). Why God permits these demonic manifestations is a bit of a mystery. In either of these cases (obsession or possession), the devil has to work his damage by disrupting human nature, the normal functions of the human person. This is why mental illness (a natural disruption of human nature) often has characteristics that also appear in the case of demonic activity (supernatural disruption of human nature).
The foregoing was excerpted from:
https://catholicexchange.com/differences-demon-possession-mental-illness-depression/
The convoluted explanations needed to justify Catholic doctrine and the fancies about Mary just go to show how little basis in reality they have.