Michael's true love was Appolonia. She would have been the essential Italian mobster's wife too. Like a Mama Corleone.
I don't believe that Michael loved Kay in the same way after falling in love with Appolonia. He was hit by the thunderbolt with Appolonia. He wasn't with Kay.
Although I think that he loved Kay in a different way, he was not as passionate about Kay as he was with Appolonia.
If he really and truley lved Kay the way that he loved Appolonia, then he would have never waited as long as he did to contact her after returning from Sicily.