There are plenty of words and phrases that can be used to express uncertainty or doubt without using the phrase "not sure." Some common alternatives include "uncertain," "unsure," "indecisive," "ambiguous," "undecided," and "hesitant." Other options might include "doubtful," "wavering," "unconvinced," "unresolved," or "undetermined." Each of these words and phrases carries slightly different connotations that can help convey the specific nature of the uncertainty being expressed. No matter which synonym is chosen, the goal is to communicate a sense of unclear or unsure feelings without resorting to the same old tired phrase every time.