If you are disillusioned, you once believed someone was good, or an idea was right, and that belief has broken. The feeling is not a quick sulk. It is what is left when the story you lived by no longer fits. People say disillusioned with or disillusioned by: disillusioned with politics, disillusioned by a teacher they trusted. With points at the thing you no longer trust; by often points at the person or event that opened your eyes. It is the usual everyday form — more common than the verb disillusion in speech. News and essays love it for voters, workers, fans. Though actually, it does not mean you now hate everything. You can still care. You just do not swallow the old pretty version. It is an adjective here (and the -ed form of the verb). A disillusioned smile is tired and knowing, not surprised.