Okay, not that I'm obsessed with Ghost & Mrs. Muir...
But I am. So I can tell you that in the beginning of the movie, when she speaks about her husband, it is clear that she became disenchanted with him. In the book, it is even more clear that she had a rapid courtship, and while he was a good husband, he wasn't a great one. He wasn't frugal with money, he wasn't overly smart, and the crush she had on him disappeared, leaving only duty in its wake.
She was a dutiful wife and widow, but their marriage was a dutiful one, not an epic romance. In the book, it's implied that she never expected to have a great love, since she had a "small life" filled with duty and respectability. She mentions in the movie that he gave her a daughter, and she "will always be grateful to him for that." As if she won't be grateful for much else.