。
Honestly, I don't really feel like it.
Literal
Actually feeling [subject-が] not-advancing [explanatory-んだ].
気がすすまない ('not feel inclined, not be in the mood') — 気 ('feeling/spirit') + がすすまない ('doesn't advance'). A natural way to express reluctance without being bluntly negative. んだ adds explanatory tone — the speaker is confiding.