God does NOT have one set person for us to marry. Rather, He influences our coming into contact with potentially suitable partners. We must have our senses and our hearts open to who these potential future spouses are, and take the actions necessary to discovering which is the one that the mystery of love will work in our hearts. How God brings two specific people together is a mystery. But we are very much involved, and it is ultimately our decision.
The free-will consent of both persons to each other on the day of the wedding is what confects the Sacrament of Holy Matrimony. The "free will" means it is our choice, and God then blesses the decision. In this decision is also the forsaking of all others. That means there could have been someone else, but this is your choice for the rest of your life.
It's a mystery how God is both an influence and also a spectator. He is very much involved in helping you find someone, but He awaits the decision of the two he has helped. Therefore, unfortunately, it does happen that, because of free will, two people that God helped do NOT come together in marriage. Is this the end for either? No! God continues to work because the vocational call is too important. There can be someone else willing to make that commitment (free-will consent) to love for a lifetime.
As long as there is free will, there will always be marriages that do not happen that should have. I do personally believe that a person can miss out on their best hope for marriage by their free-will decision in the past to say no to someone who was a good, suitable partner. It does not mean it is over, it just means it might become that much more difficult, or might take longer than we hope, etc.
God is not going to just send someone "despite" our efforts, but rather in "conjunction" with our efforts. We have to get into environments that make sense for meeting good, single Catholics. Otherwise, God's job is much harder. It's not impossible, but it is harder. For example, if you live in an area where there are hardly any Catholics, and you are unwilling to move or unwilling to travel to meet someone, then you are forcing God to perform a miracle. We have to do our part. God helps those who help themselves.
Let us not blame God for not bringing us "the one". It is probably our own fault, whether it is because we are too busy to make time for dating, or our expectations are too high, or we want everything to be perfect, or we want marriage on our own terms and time frame. If you are to be married, then you have to get on with it, and do whatever is necessary to find a suitable partner.