A mistress in a dream is read as secret desire or a divided heart. Tradition also sees it as something that draws your attention away from a commitment — a passion, a project, a hidden wish.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.