Can you do it successfully? Yes, yes, yes, yes, yes.
Is it hard? Yes, yes, yes, yes, yes.
DH and I have been married 13+ years and dated for 5 before that. We now have 3 kids with #4 on the way. He has been a life-long, dedicated Presbyterian while I consider myself an orthodox Catholic. Long before we married, we agreed that
if we were to marry (and this was a condition for me), our children would be raised Catholic. This was-- is-- extremely important to me, and he knew this and not only respected our agreement, but has been truly supportive in the raising of our kids Catholic.
In my opinion, your and your fiancee’s attitude about the whole thing is paramount. Is he not only tolerant but **supportive **of your faith and spirituality? Is he willing to attend Mass with you? Similarly, are you willing to pray with him in ways which are comfortable to him? Are you willing to support him in his faith journey as he attends church with another congregation? Also, you must both enter the marriage with no expectation of the other converting at some point. You must be willing to accept the real possibility of your being married to a devout Protestant for the rest of your lives!
My DH was very active in a local Presbyterian congregation for many, many years. Until our first child was born, we attended 2 services: Mass at 9 AM, followed by worship at his church at 11:30. Sundays were not what I would call relaxing. He was a deacon and spent many evenings at his church, involved in various ministries and church activities. I had to learn to be supportive of this, as I truly believe it brought him closer to the Lord and was beneficial for his soul. At the same time, he attended social functions at my parish with me and we became comfortable with and a part of both communities. While there were a few isolated (and memorable!) exceptions, almost everyone at his church was supportive of our relationship. While we had plenty of theological discussions, they were mostly respectful and never did I experience the Catholic-bashing so many have. I really felt welcomed in his congregation. In contrast, some Catholics at our parish were not so charitable
Once our babies came along, going to 2 services became impractical. I stopped going to his service, but he continued to come to Mass with me, in no small part to help with the kids. He is encouraging of them when they attend CCD, he asks questions, he encourages them to be respectful during the consecration. He prays with them in the evening and makes it clear the importance of God in his (and our joined) life together.
I could go on about how I believe this relationship, our marriage, has led us both closer to Christ. Have I ached for him to enter the Church? Of course. I understand John 17 better than I ever, ever hoped to. Do I yearn to share the Eucharist with him? Absolutely. It is one of the great heartaches of my life. Do I feel blessed to have him as my spouse? Has he brought me closer to God? Without a doubt.
Sorry for rambling, it’s a topic about which I am passionate.
(In the interest of full disclosure, I should mention that my DH did enter RCIA this fall-- Praise God!)