if this does not mean "I lost heart" in the sense of "I lost courage" then what does it mean?
Is the lost heart the author's own?
Or that of some animal, which he might have been hoping to eat, or experiment on perhaps, later?
Or is he lovesick in some way.
My guess is that it refers to how Duolingo lessons used to work, where one would begin a lesson with three “hearts”, each error made would result in the loss of one of these hearts, and making an error with no hearts remaining resulted in no experience points being gained for the lesson.