"If you had any advice on Catholic/non-Catholic relationships (i.e. discerning whether your differences on religion are an insurmountable obstacle to marriage and raising a family) I would really appreciate it."
Man oh man, I could talk about this for six years straight and not even scratch the surface! I actually had an article appear in my local Catholic paper on this exact subject, so I'll link to it here. The journalist who wrote it did a really nice job of distilling my ramblings into positive, practical statements. I love what she did with the article.
If I were to sit down with someone contemplating marriage to a non-Christian, though, I'd have a few things to say that aren't so positive. The reality is that marriage is hard. H.A.R.D. HARD. It takes a committment to the relationship through good times and bad, it requires personal sacrifice and compromise, and you will have to constantly think about it and work on it for the rest of your life.
It's worth it, 100%. I wouldn't trade my husband for anyone, even a practicing Catholic, and the rewards that I get from being in a committed relationship, from bearing children, and from joining my heart to another are indescribable. Before I married, I had serious social and emotional problems due to my upbringing. Those have been healed through the power of my husband's love and the weight of our relationship grounding me to a secure foundation. I struggled right up until about a year ago with the notion that I might have been meant for a different vocation, but I've come to see that it's through marriage and motherhood that God is fitting me for heaven.
Specifically to those who are considering marriage to a non-believer, I would say:
1. It's going to be harder than you think.
I am continually surprised by how differently my husband and I see the world. Even though he and I share the same basic values, our reasons for having those values are fundamentally different, and every now and then those differences make a big difference! We can approach an issue with the same intention in mind, and yet come up with opposite solutions because our fundamental understandings don't line up. To cut to the chase, as a Christian I am fundamentally hopeful. It is the mark of my relationship with God that I believe He is in control, and as He wants good for us, we will ultimately encounter good by following and trusting in Him. My husband, as a pragmatic realist, is fundamentally suspicious and fearful. I know he would object to that characterization, but I believe it is true. There is nothing for him but this life. Any moment of unhappiness or strife that it not geared toward a future payoff has no benefit to him, so his primary interest is in acting in a way to ensure that this life he has is, and remains, as comfortable and secure as possible. That makes him an absolutely terrific provider, but it means the things I consider most important (instilling faith in our children, giving to the poor, opening our hearts to others) are to him only extras once the real business of life is achieved (financial security, stress reduction, professional fulfillment.) I am looking at this life as the first step to eternity. He is looking at it as the only existence that we will ever have. We don't even see the same ramifications to each of our choices, and that divide is something that we cannot overcome through dialogue or compromise. We are working towards different goals, and I didn't fully realize until well into our marriage just how much that would impact our relationship.
2. You must marry your spouse as he/she is, without any expectation for change or improvement in any area.
This actually is good advice for any marriage, whether you share a faith or not. It's true that some change WILL occur. People are not static beings. Our circumstances change, and that makes us change, too. But if we expect certain things to improve in our spouses, we are very likely to be disappointed, especially if we are expecting conversion. I've seen it happen to friends of mine, but it hasn't happened to me. If I had expected that by the time we had two kids, my husband would be a Catholic, I'd be beside myself with frustration right now. I had to decide when we married that I would be happy going the entirety of our lives together without his conversion at any point, and I decided yes. The person that he is still appeals to me as a mate. His character is such that I would choose to marry him again if I encountered him in any place or time. And it is his character that I love, and will continue to love, no matter what the state of his soul.
3. You must be able to be authentically, fully, deeply Catholic without an Earthly support network, and even in the face of hostility or contempt.
As Catholics, we always have the support of heaven in our quest to serve God. In a mixed marriage, however, there may be times when practicing our faith is very difficult, and we won't have any Earthly support. When I first went to visit my husband's family, for instance, I had to figure out how I was going to get to Mass on Sunday. Their tradition was a walk into town for brunch at their favorite breakfast place. Not only would I have to miss this, but I had to ask them to use their car to drive myself to a strange Church I'd never been to and sit there, alone, to worship. If they had been strictly against it, they could easily have prevented my attending Mass. They could have laid a guilt trip on me for disturbing their perfect morning, or for choosing some archaic ritual over getting to know them better. They are wonderful people, so instead they delayed their walk, gave me the car, and waited for me to return so we could go to brunch together. Not everyone is so accommodating. There are many people out there who find religion offensive, and if you are marrying into a family who has had bad experiences with religion itself or people of a particular religion, then you need to search your soul and determine how much you are willing to fight for your faith. Even my loving, generous in-laws still have certain hang ups about the role of faith in culture and politics. We've had several difficult conversations about the sexual abuse scandal, abortion, the Pope's decision to reopen the Tridentine Rite, and the advice of the USCCB on voting. When I visit them, I have no one with me to help me defend the faith. It's ALL on me. That goes for instructing my children, too. Be prepared to feel lonely at times, and most importantly, be prepared to love your new family even while they're challenging you on the core of your beliefs. Especially if they've had bad experiences, it's now up to you to show them how God's grace works within a person who believes. Trust me, they will be watching you.
4. You MUST know your faith.
Education is the primary tool you need in a mixed marriage. If you know God, you will love Him, and know how to serve Him. Without truly knowing your faith, you will not only be unable to defend it against argument, but you will also be missing out on the beauty and depth that can sustain you through difficult times. For instance, many Catholics are unaware of the Church's position on birth control. This is mind-boggling to those who were well-catechized, but the fact is most catechism classes and Pre-Cana preparations say NOTHING about family planning. I myself only found out about NFP when my new boss talked to me about he and his wife's journey to embrace life in the context of their marriage. I'd been married for 2 years at that point, and had been using a barrier method of contraception that whole time, because I thought, erroneously, that as long as no fertilization took place, the Church permitted birth control. Once I discovered the truth of Church teaching, I examined it and brought it to my husband for his acceptance. He wasn't thrilled, but he agreed. Until that point, I had been dead set against having children at all. I thought I would be a bad mother, I worried my child would inherit dangerous personality traits from my family, and I feared I would make too many mistakes and they would grow up to hate me. God took that fear and made me face it. We would not have conceived our firstborn if we were not using NFP, not because the method failed, but because we had no compelling reason to abstain during a fertile time. God blessed us with NFP, as he has blessed countless other Catholic couples. Before I knew the truth of Church teaching, I had missed out not only on an opportunity to witness my faith to my husband, but also on the joys and challenges of parenting. Which, as I said above, I now realize God planned for me from the start.
God calls certain people to this kind of marriage. I would never counsel someone who is in love with a non-Catholic that they should end the relationship simply for that reason. The Church herself does not prohibit marriage between peoples of different faith, or no faith, precisely because what God joins, no one on Earth may separate. If God has led you to a non-believer, then listen to His call. He puts people in your life for a reason.
I have challenges in my marriage, but they are no harder than those faced by many of my friends who share a faith with their spouse. I am blessed to have a husband whose job I am proud of, who shows me respect and faithfulness and generosity of spirit with everything he does. I have Catholic friends whose Catholic husbands aren't open to life, yet my atheist husband is! True, he doesn't help me instruct the children in the Catholic faith, but then, he's never once objected to it, and even tells my son that he has to go to Mass with me.
My basic advice is this: Make your decision based on the character of the person you want to marry. If your choice happens to be of a different faith, or no faith, then arm yourself with a strong love for God, a deep knowledge of your faith, and a willingness to endure hardship for His sake. God will do the rest.