Temporally, the gap between the now and the then. (cf Fritz Perls). There is an orientation to the future – contrast this with melancholia, and its ‘if only’.
Structurally, a herniation of one’s fantasy by the real. As the perforation gapes, panic sets in.
Dialectically, the lack of a lack. The negative of lack has a positive correlate – this is the object, or, better yet, desire itself. Whence the interchange between repressed, neurotic desire and anxiety (cf Hans Sachs), and the close affinity between phobia and perversion.
Experientially, to be caught in the matrices or interpellation of the Other’s desire. Cf Turgenev’s novella, First Love, in which our young protagonist gazes, unrequited, at his beloved neighbour from afar, only for her to suddenly gaze back.” ‘Que suis-je pour elle?’ I thought (God knows why) in French.”