In what sense are “memories forever” in Lois Lowry's The Giver?

In Lois Lowry's The Giver, memories are "forever" in the sense that they must be held by at least one person. They will not cease to exist. Released by one person, they inevitably will attach themselves to others. The community has found some means to place all memories in the Giver, where they stay until the Giver passes them on to the Receiver, who becomes the new Giver. Whoever is holding the memories must remain in the community. If the person who holds the memories goes Elsewhere, the memories return to the community. The Giver explains to Jonas that this "would mean that the community has to bear the burden themselves, of the memories you had been holding for them" (155). As Jonas begins to understand the horror of this community, he wants to leave, but there is no one ready to replace him as a new Receiver and the Giver cannot take back the memories he gave to Jonas. As a result, the permanence of memory creates a situation in which Jonas's leaving is likely to overwhelm the community with all the memories he already holds.  
This idea, the permanence of memory, seems fanciful, but Jung thought that humanity had a "collective unconscious," and people who suffer from post-traumatic stress disorder (PTSD), suffer from the problem of traumatic memories they cannot make go away. For Lowry, this story is an exploration of how we might be mere automatons without memory, unable to properly feel, reason, or have control over ourselves.