The antonyms for the phrase "more second-nature" may include terms like challenging, unpracticed, unaccustomed, difficult, uneasy, and unfamiliar. While something that is "more second-nature" comes naturally and without much thought, these antonyms imply a sense of discomfort or unfamiliarity. For instance, if you are learning a new language, speaking it may not come "more second-nature" to you, and you might feel challenged and uneasy until it becomes more familiar. Similarly, taking up a new hobby or learning a new skill may pose difficulties in the early stages until they become more practiced and more second-nature to you.