What does Belle say has replaced her as the love of his life?

When the Ghost of Christmas Past takes Ebenezer Scrooge back to his past to see the end of Scrooge's relationship with Belle, the spirit shows Scrooge two specific scenes involving the woman.  In the first, Scrooge is "a man in the prime of his life."  However, his face had already "begun to wear the signs of care and avarice. There was an eager, greedy, restless motion in the eye, which showed the passion that had taken root, and where "the shadow of the growing tree would fall."  This description is particularly useful to understanding Belle's words to Scrooge, as she says "[a]nother idol has displaced me."  Scrooge inquires as to what idol that is, and she replies "a golden one."  By this, she means that Scrooge's attention has turned to money, and the accumulation of it.  This is the reason why the signs of avarice are already upon his face.  Belle goes on to say:

Our contract is an old one. It was made when we were both poor and content to be so, until, in good season, we could improve our worldly fortune by our patient industry. You are changed. When it was made, you were another man. 

This fully clarifies the symbolism of the idol, as well as the change in Scrooge.  When Ebenezer and Belle met and fell in love,  they were both poor; they shared a worldview regarding their situation and took solace in the shared belief that there are more important things than money.  However, since that time Scrooge changed.  In the moment of the memory, money is not only the most important thing to him, but the only important thing.  It is because of this that Belle ends their relationship, leaving Scrooge completely unfettered to fully devote his life to the pursuit of money.