(From what I remember because I read this book like, months ago)
He does truly love Mildred, but she does not truly love him. He questions whether or not she remembers how they met and she does not recall it. She is also very careless when it comes to Montags well-being to the point where she eventually ends up turning him in for the stolen books and killing herself.
(I would suggest if you know any better than I do to change this answer a bit because I read this book back at the beginning of this semester)