The Gospel gives us the Bible’s formula for a true marriage: “the two shall become one flesh.” Of course, this union doesn’t come about automatically, since each of us like’s to have his or her own way. Perhaps you have noticed that. The marriage ceremony can resemble the bell beginning round one of a fifteen round fight.
The fact that Jesus said Moses was allowed to alter the rules because of the hardness of people’s hearts tells us that the rules are not unalterable.
Jesus said divorce is permissible when the marriage was unlawful. That is the basis on which the Church grants annulments. For one thing, a lawful marriage is one into which each party freely commits him or herself. An annulment may be granted when the Church, examining all the evidence, decides that one of the parties was not free. That would be the case if the bride’s father had a shotgun at the groom’s back. It could also be the case when there was an invisible shotgun involved, as when one party was not completely sane.
Divorce, but not necessarily with the freedom to remarry, can be the best course when staying together brings great unhappiness to one party.