The last time we heard Gwen Stefani sound this sad, it was 1995 and "Don't Speak" was climbing the Billboard charts. Her latest tune, "I Used to Love You," might be even more heartbreaking. Especially when you consider where the emotional lyrics are coming from.In the wake of reports that her divorce from Gavin Rossdale has officially reached a settlement, the singer appeared on The Ellen DeGeneres Show, where she performed the new track that echoes all the heartache that generally surrounds a breakup.It's hard not to assume that "I Used to Love You" was written about the breakup of Stefani's marriage after 13 years. Though the singer released the official video last week, there's something even more raw about watching her sing the track on Ellen.Before the number opens, Ellen shared that Stefani revealed the track to her only two weeks ago, playing it for the host in her bathroom. The pared-down performance strikes the perfect balance of poignant and pop.