What is the truth about forever, anyway?
The first one to talk about "forever" in The Truth About Forever is actually Kristy. Let's take a look:
"If your forever was ending tomorrow, would this be how you'd want to have spent it?" (7.210)
This really gets Macy thinking, about her life, her future, and about, well, forever. At first, it's ultra negative, just like her outlook: "Never would forever, with all its meanings, be so clear and distinct as in the true, guaranteed end of the world" (13.14). But soon, the thought of how she wants to spend her forever starts influencing her decisions. When she quits the library, it's because "[i]f this was my forever, I didn't want to spend another second of it here" (14.105).
Good call, Macy.
And then, of course, Macy decides to follow her heart, and run after Wes (the one who introduced her to Truth, by the way):
Everyone had a forever, but given a choice, this would be mine. The one that began in this moment, with Wes, in a kiss. (21.90)
In the end, she realizes that the truth about forever is that forever is always happening. It's all around you, and you get to choose how you spend it. And Macy chooses to get past her grief and live her life—with Wes right beside her.