Or, do I use propositions in my mind, to become conscious of my state of mind?
Surely both, since even without Friday a Crusoe castaway is not going to lose capacity to know his own states from lacking anyone to converse with. And in social settings, we can receive feedback from others about what they interpret as anomalies in speech and body behaviors that we might not otherwise realize. Obviously, though, a person must first assimilate the "public utility" of language early in life to be so enhanced by its private employment (a wild child falls back upon pre-human alternatives).
And on that last note: Animals and babies would seem to require a manner of sensory/intuitive intentionality not dependent upon language. This doesn't necessarily rule-out intellectual support, however, since there could be a wider range of more precise emotions serving a concept role with some species (both internally and as external body expressions). Also, even small-brained nondomesticated turkeys apparently have a vaster "vocabulary" of communication signals than we'd usually imagine, with these likely innate. So a precursor to language may after all be carrying part of the load for discrimination and "aboutness" in some creatures.