Learn the Catholic Faith: May a husband and wife who are always fighting get a divorce?
The Church has great respect for the ability of a person to keep a promise and to bind himself in lifelong fidelity. She takes people at their word. Every marriage can be endangered by crises. Talking things over together, prayer (together), and often therapeutic counseling as well can open up ways out of the crisis. Above all, remembering that in a sacramental marriage there is always a third party to the bond Christ can kindle hope again and again.
Someone for whom marriage has become unbearable, however, or who may even be exposed to spiritual or physical violence, may divorce. This is called a "separation from bed and board", about which the Church must be notified. In these cases, even though the common life is broken off, the marriage remains valid. Indeed, there are also cases in which the crisis in a marriage ultimately goes back to the fact that one spouse or both was not eligible at the time of the wedding or did not fully consent to the marriage. Then the marriage is invalid in the canonical (legal) sense. In such cases an annulment procedure can be introduced at the diocesan tribunal.
(From the Catholic Youth Catechism question 269)