In this opening paragraph Barthes is accounting for the method of this strange book. He is not going to produce a theoretical discourse which analyzes other discourses of the type “lover’s discourses.” He will not do what theoretical discourses do: find the commonalities and differences, categorize and dissect, pretend to view from the outside. Rather, he will write what is itself a lover’s discourse. For this reason, he refers to his writing as a “primary language” rather than a “metalanguage.” A metalanguage pretends it is not part of the discourse it interprets (I say ‘pretends’ because this is impossible) and so to reflect on it from the neutral terrain of an impartial observer. Of course, a “primary” language might be a sort of dream as well - this would be a language free of any interpretation or auto-critique. Nonetheless, Barthes claims to be writing such a discourse.
For sources on metalanguage, I would recommend some of Barthes’ contemporaries, for example Jean-François Lyotard’s The Postmodern Condition: A Report on Knowledge or any of the many relevant works by Derrida. “Structure, Sign, and Play in the Discourse of the Human Sciences” (from Writing and Difference) and “Differance” (from Margins - Of Philosophy) would both be relevant introductions, while the opening piece from Dissemination whose title has been somewhat playfully translated as “HORS LIVRE: OUTWORK HORS D'OEUVRE EXTRATEXT FOREPLAY BOOKEND FACING PREFACING” would be more thorough.
His reference to a “symptomal subject” is a reference to psychoanalysis. Psychoanalysis produces just such a metalanguage about the lover - his or her passion is dissected as a symptom of various neuroses, inhibitions, childhood traumas, etc. If you are interested in reading more by Freud “Inhibitions, Symptoms, and Anxiety” might be relevant, although this reference is broad enough that you could start anywhere.
His use of “unreal” seems more idiosyncratic to me. He glosses it on the previous page (in my version there is one paragraph before the sentences you cited) as “exiled from all gregarity”. It seems this refers to the marginalization of the passion of a lover by the various forms of theoretical discourse, all of which Barthes is eschewing. Unreal means just this - such feeling is banished from social reality. If it is related at all, this is opposite how Lacanian psychoanalysis would use the term “Real,” so it seems to me that there are no sources to pursue here.