The speaker is watching a relationship begin to slip away before it has actually ended. Someone they care about is already talking to a new person, described as pretty, funny and full of witty conversation. The speaker insists these are "just phone calls and text messages," but that word "just" sounds like an attempt at self-reassurance, and it doesn't really work. Every ring still hurts.
The feeling here is anxious anticipation. The speaker can "feel it looming just over the horizon," like bad weather approaching, and the physical details, an ache in the heart and a jolt down the spine, show how much the dread has taken over the body. The last line, with its "shiny new toy," is bitter and self-deprecating. It suggests the speaker feels like something old and disposable, which fits the title and the speaker's name, "Ancient Artifact." The mood is tense, wounded and resigned, as though the speaker has been through this before ("once again left behind").