In "Fahrenheit 451" how does Granger's message help Montag understand and deal with the way he feels when he thinks of Mildred's death?

Despite the difference that Mildred and Montag had, and the fact that she turned him over to the authorities, it is still Mildred that he thinks about when he leaves the city behind.  After the bombs actually hit, Granger doesn't say much--none of them really do.  But before they even hit, Granger helps Montag to deal with the fact that he is leaving Millie behind.   He tries to help him have a bit of perspective.  To do so, Granger shares memories and wisdom that he received from his grandfather. His grandfather was a great man who impacted everyone's life that touched his own.  And, he said that after you are gone, nothing really matters

"so long as you change something from the way it was before before you touched it into something that's like you after you take your hands away."

Granger, in his own strange way, is trying to tell Montag that even though he left Millie behind, she is still with him, because she impacted his life, and he impacted hers.  Granted, the impact in Millie's case wasn't necessarily positive or pleasant, but, the time that they spent together will always be there.

After the bombs hit, Granger offers comfort to everyone as he tells the legend of the Phoenix, a bird that built up an altar of wood and burned itself up, only to be reborn again. He says that is what happened here--their civilization is in ruins, but at least they can rebuild, and hopefully this time, they can do it right so that they don't repeat the same mistakes and end in the same way.  I hope that those thoughts helped; good luck.