I know this isn't the book you're thinking of, but the first that comes to my mind is The Constant Princess by Philippa Gregory, a treacly historical romance novel about Catalina, the Spanish Infanta, daughter of Ferdinand and Isabella who becomes known as Catherine of Aragon when she marries Arthur, Prince of Wales. Negotiations for their marriage began when they were toddlers, in an effort to forge an Anglo-Spanish alliance against France, and agreed upon when they were tweens. She was shipped off to England a couple of years later, along with her enormous dowry. The book details how she is separated from her life, including even from the name she was known by, and treated as an oddity, made to feel as a stranger in a strange land, if you will. Arthur was a year younger, and hadn't yet hit a growth spurt so he was smaller than her and very much still a boy physically and mentally. He was so young he was below the age of consent (for males) and their marriage only possible by a papal dispensation. The book doesn't show it as being love at first sight, but rather fear. They detested and resented one another at first. He behaved obnoxiously and thought her peculiar. She as overwhelmed with loneliness. They were both not just awkward with but terrified of physical intimacy. Over time though they did develop a true friendship; it was like they'd been marooned together in this bizarre island of young royal marriage. And then as Arthur physically grew, so did the romance between him and Catherine. He actually was kind to her, and showed her decency and love. When he died, aged 15, she was legitimately heartbroken and mourned him. Then to compound her grief, she couldn't return home because England refused to return the dowry to Spain, and Spain refused to take her back without the dowry. So she was forced to remain in England and wait for Arthur's bratty younger brother, the heir apparent, to reach the age where he was eligible to marry her. If I remember correctly (I read this when I was 12) in the book she and Arthur did consummate their marriage, to the satisfaction of the adults intent on confirming that at the time. Then after his death and the decision that she would be betrothed to Arthur's brother so the dowry and alliance could be kept, the same adults convinced her she was merely confused, that it wasn't a proper consummation, and therefore she was eligible for the second marriage. We know how King Henry VIII treated his wives....
Of course the writer took many dramatic liberties with the book, including with the timelines. In real life the two began to know one another first via letters they exchanged that were written in Latin, a common language for them. They had been able to grow some affection before meeting on their wedding day, as evidenced from those letters. He did seem to actually have a sweet and honorable nature, in contrast to his brother.
I'll think on the book you're referencing to see if I can figure it out.
