Marriage means living with another person, and living with another person is a constant challenge to better yourself. Because when you don't live with a person, things you say or do, or don't say or do, and vice versa, don't matter at the end of the day, but when you do live with a person, everything matters.
Marriage is a wake-up call. The perfect partner does not exist, no matter how "made-in-Heaven" s/he may seem at the start. Only one Being is perfect, and if you are looking for perfection, I suggest you become a hermit and spend the rest of your days in a cave, communing with God and nature. Not your vocation? Not surprising, since most people need to be challenged in their path to sainthood.
This isn't to say that I do not believe in the idea of soul mates. I do believe soul mates exist. I do not think that every one meets a soul mate in this life, and even less end up married to one. Some of the well-known saints had soul mates with whom they kept up a lengthy correspondence, while still remaining celibate. For example, St Teresa of Avila in her later years was very spiritually close to a much younger St John of the Cross, and St Francis and St Clare of Assisi were equally close.
Certainly, it helps to be in a marriage with that person who, when you first met, seemed made in Heaven, just for you. It helps when you are with a person who has the same interests, a similar background, the same desires. It helps to be with someone whose mind you love; whose soul inspires you; someone who continues to amaze you, whose ideas are so close to yours that sometimes it seems that you are of one mind.
But even then, marriage will still be a battlefield. There will still be misunderstandings and shortcomings, and you will still be put through the fire of testing that will gradually forge you into the person you were meant to be.
Neither of you is perfect and you will challenge each other on the road of life to better yourselves. Only you must accept the challenge. Marriage is a battlefield, but you are in this together. You will fight the demons that plague you; pride, prejudice, laziness, procrastination, lust, distrust, domination, selfishness. Marriage is a constant battle against these things, and requires the proper armour and weapons; faith (in God and each other), compassion, communication, trust, humility, empathy, Truth.
Marriage between two people is that sacrament, that imitation of the relationship God desires to have with each one of us. With every "I'm sorry", and every "I forgive you" you bend yourselves a little closer to the shape that you were meant to be. Sometimes the battlefield is bloody. Love hurts. Hearts bleed. But you are still in this together and the only way to come out stronger is to endure the fire and the pain.
This is why even the worst marriages have still produced saints. Even married to abusive, domineering men, Saint Rita and Saint Monica still answered the challenge to better themselves through trial. These women went through kiln and fire, battlefield and blood and came out strong and pure and beautiful. They had every reason to leave. They may not have had the option of physically leaving or obtaining an annulment as they surely would have in our age, but even if they had no choice in their circumstances, they still had a choice in how they dealt with the circumstances. They could have become bitter, abused women, but instead of mentally "leaving" their husbands, disconnecting from them, returning hurt for hurt, they chose to love them (even though they may not have "liked" them much.) They rose to the challenge to better themselves. They sought the love they needed in a relationship with God instead. The end result is that their husbands converted (albeit on their deathbeds).
By this, I am not advocating staying in an abusive relationship, far from it. I'm also not saying that there are never cases when separation and annulment are valid options. I am just saying that even in the worst situations, whether it be in a difficult marriage, a difficult situation at work, or anywhere else, you can still choose to rise to the challenge and become a better person, even if it seems that the people around you are uninterested in following suit, and are unwilling, themselves, to make the effort to reconcile. Sometimes we rise to the challenge and we go to battle and we lose the battle. Sometimes, despite our best efforts, we lose. When a marriage fails, we can let even that challenge us to become a better person.
So your marriage isn't perfect? Welcome to the real world. No one's marriage is perfect. We are all sinners. The question is, am I going to get out my battle gear and rise to the challenge? Am I going to pull out my sword and pour myself into this thing despite the blood and sweat and tears? Am I going to let my marriage make me a better person?
Saint Michael the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil. May God rebuke him, we humbly pray: and do thou, O Prince of the heavenly host, by the power of God, thrust into hell Satan and all the evil spirits who roam throughout the world seeking the ruin of souls.