Is it permissible to continue supplicating that a girl I know has become engaged would love me, especially since Allah has answered my supplication in knowing that she loves me?

If the intention of the supplication is to marry the woman and to make Allah place your love in her heart so that she agrees to marry, then there is no objection to that, as long as she is not engaged to another person. This is because, in such a case, the supplication would be considered an act of spoiling her engagement, which is impermissible.

However, if the intention is merely love without marriage, then that is not permissible. Love sickness can be cured, and a Muslim should not confine himself to one woman, as there are many women. He should supplicate to Allah to grant him what is good for him.