In addition, Belle also seems to intuitively understand that, if she marries Ebenezer, he will never truly value her. He asks her in what way he had ever sought to be released from their engagement, and she says,
"In a changed nature; in an altered spirit; in another atmosphere of life; another Hope as its great end. In everything that made my love of any worth or value in your sight."
Ebenezer has not explicitly stated that he wishes to end his relationship with Belle, but the way he seems to privilege money over everything else—including her—makes Belle realize that, in marriage, he will not find her love to be a worthwhile or valuable treasure. He has eyes only for one treasure: gold. I think it is fair to extrapolate an idea of Belle's own sense of self-worth; Ebenezer may not know her value anymore, but she knows her own. She understands that they now have different goals in life, goals that are incompatible. She will not settle.
Furthermore, she tells him, "I release you. With a full heart, for the love of him you once were." In other words, she no longer loves him. She says that she loves the man Ebenezer used to be but is no longer. He has changed too much, while she has remained the same. It was not this new Ebenezer with whom she fell in love, and, in parting from him, she seems to admit it.