If you ever understand the science of love and falling in love, you will make millions! No one can make themselves fall in love with someone he/she doesn't; by the same token, you can't make yourself not love someone you do. If we could turn it on and off like that, the wonder of love would be lost. There would be no challenge or conquest at all to the "battlefield" of love (Pat Benetar referred to it in her famous song).
Angellica is a courtesan who makes her living and satisfies her desire for a lifestyle of wealth and comfort by marketing her body to rich men. This lifestyle is a matter of survival for her, so to fall in love with anyone would be the end of her life as she knows it. She not only enjoys the wealth of her lifestyle, but also the control she has over these men. If she fell in love, the shoe would be on the other foot...she would no longer be in control. However, as I mentioned above, we do not get to choose with whom we fall in love or when it will happen. Quite possibly, she had no choice in the matter when she falls for the Captain. Fortunately for her, Don Antonio takes her in so that will no longer have to advertise her wares to make a living.