This is another uncomfortable Gospel reading, but don't let that put you off. The Good News of Jesus Christ is supposed to make us feel uncomfortable - which is certainly not the same thing as unconsoled - and so our reaction is natural and good. Persevere.
Moses' prescriptions on divorce, as quoted by the Pharisees, are found in Deuteronomy 24: 1-4. If you haven't already read it, go and do so now, but be warned that it makes for even less comfortable reading than the Gospel.
The point of this Old Testament passage is not directly to do with the permissibility and procedure of divorce. Rather, it takes for granted that writs of dismissal were served to wives, who were thereby divorced, and serves to forbid husbands from taking their wives back if they have been married and divorced again in the meantime. In other words, in the days of Moses, it seems that while marriage was not inescapable, divorce was.
Jesus changes things. He makes marriage something new, something better, something holy, wrought by God - and far from doing so arbitrarily, He reminds us that this had been God's intention for marriage from the very beginning. As Chesterton put it, He made married love romantic by making it irrevocable.
What does this mean? Well, for one thing, it turns the Mosaic view of marriage on its head. Because marriage cannot be broken, divorce can be. In other words, if a married couple separate, and then both change their minds, there is no longer any circumstance under which they would not be permitted to return to one another. To apply this directly to the passage in Deuteronomy, whereas once the wife could not be readmitted, now she must be, because the bonds that tie her to her husband can never be severed, no matter what faults the parties commit.
The marriage covenant, then, is a covenant of endless mercy, of unlimited second chances, of the sacrifice of forgiveness and ever renewed love, until death do us part. It is therefore no coincidence that marriage is likened to the bond between Christ and His Church in the writings of St Paul and in the Book of Revelation. No matter what the faults of her members, the Church is irrevocably bound to Christ by a wedding band forged in the very Heart of God. Jesus will never - can never - turn His bride away, as often as she turns back to Him.
As it is true for the Church, so it is true for her individual members. You cannot remove the indelible character of divine adoption bestowed on you in Baptism, for all you can ignore and reject it. Christ has welcomed you into His Body and sealed the bond with His Blood, and you can never run far enough away from Him that He might forget the promises He made you (Psalm 138 ).
It has always been the case that human relationships can break, and some good people find themselves in difficult or irregular family circumstances. The world is full of broken homes and broken hearts, and it is full of people who have found love in circumstances which they think put them out of reach of God's mercy - but there is no such circumstance, because "where sin abounded, grace did more abound" (Romans 5:20). Our Lord's words about marriage in today's Gospel can be difficult to swallow, and there is no doubt that they are challenging, but it is precisely in the challenge that we find the consolation, because by asking you and me, weak and fallible as we are, to be faithful in sickness and in health, He is guaranteeing that He, the holy and mighty One, will be faithful to us come what may. So many of His flock feel estranged from the Church, and from hope itself, because of their family circumstances, and often there is no quick fix, no way except the way of the Cross. Nevertheless, it is true for every single person alive today, for every person that will ever live, that however far from God and His Church they might be, there is always a way back.