This just totally hit me like a fright train

When Snape show's Dumbledore his patronus and Dumbledore askes "all this time?" And Snape replies with "always"

Ok now think of this.

When Harry goes to into tr forrest to die..he askes Lily if they'll stay the whole time.

And remember what she replies with?

"Always"

IT TOTALLY BLEW MY MIND INTO SMITHEREENS