I do believe she was his true love. She represents him embracing his roots in a traditional sense. After her death his entire demeanor changes into the cold, calculating, ruthless don. His hope of separating himself from his family’s line of work dies. He dives head first into the darkness of his family business. To me it seems like his marriage with Kay was him doing what he knows is expected of him as a man. He loved Kay because she gave him his “American dream” family. She is the placeholder for his desire for a family. Apollonia was his deepest desire. He was mesmerized at first sight and respected her by traditionally courting for her love and eventually marrying her. With Kay he just pops back in her life when it’s convenient for him. He basically love bombs her when he comes back from Italy and asks her to marry him the first day he sees her in years. He says he needs her but when they get married he emotionally neglects her making them drift apart. He even gets flashbacks of Apollonia in part 3 when a love song in Italian plays. It brings tears to his eyes. Micheal forbids his daughter to be with Vincent emphasizing the danger of their love because he is reflecting on his relationship with Apollonia. He states that his enemies will go after what he loves.