First, Jesus said that from the beginning in marriage a man and a wife become one, and divorce is not at option for them. But then he said that because of the hardness of their hearts Moses did allow divorce and remarriage.
Jesus said, “Whoever divorces his wife (unless the marriage is unlawful) and marries another commits adultery.”
So, although divorce and remarriage is not an option, Jesus had allowed for those two loopholes. The first came when Moses allowed it because of the hardness of people’s hearts. The second loophole comes when the original marriage was deemed unlawful.
Now, the duty of self-preservation is akin to the commandment against murder. When a person is locked into a marriage that seriously threatens that person’s health the duty of preserving his or her health calls for a separation. But does it allow for entering another marriage?
As for Our Lord’s phrase “Unless the marriage is unlawful,” what does that mean?
One thing it could mean is that marriage must by its nature be a free commitment, so if a lack of freedom was there, it might never have been a marriage.
In my first fifteen years as a priest I never heard of Catholic marriage being annulled, but in 1967 a group of experts convinced Pope Paul VI that some unions could be annulled because they were not true marriages. To determine the existence or non-existence of an unbreakable bond most dioceses have set up tribunals to give such cases a competent hearing.