People who are disillusioned have lost their illusions. This is usually meant in a negative way, as disillusioned people tend to be a little bitter.
When you're disillusioned, you're wiser but not necessarily happier: you've learned from experience that life isn't always how you'd like it to be. Getting divorced could leave you disillusion about marriage and love. Finding out a teacher cheated could leave you disillusioned about education. We all are disillusioned at some point, and it's usually a bit depressing. Often, ignorance really is bliss.