In William Congreve's "The Way of the World”, what about Mirabell’s behavior suggests that he is reformed?

In William Congreve's The Way of the World
the aspect of Mirabell's behavior that suggests that he is "reformed" is that
he seems to be finally able to have feelings, true feelings, for only one woman. His
feelings for Millamant are new to him. He was once a rake who would fool around, sleep
around, be treacherous to his servants, two-faced to his peers and certainly the biggest
of the ladies' men.

However, his choice to love Millamant
results in that he now has a broken heart. No rake ever bothers being heart-broken. They
just go with the flow and live for the moment. Yet, the fact that Mirabell even thinks
of Millamant as a future wife and the fact that she is obviously the focus of his
affection is one of the biggest factors that could lead us to believe that he has,
indeed, changed.