to be one's own mistress

Webster's Dictionary (GCIDE)

1.A woman having power, authority, or ownership; a woman who exercises authority, is chief, etc.; the female head of a family, a school, etc. [1913 Webster]

The late queen's gentlewoman! a knight's daughter! To be her mistress' mistress!— Shak.[1913 Webster]

2.A woman well skilled in anything, or having the mastery over it. [1913 Webster]

A letter desires all young wives to make themselves mistresses of Wingate's Arithmetic.— Addison.[1913 Webster]

3.A woman regarded with love and devotion; she who has command over one's heart; a beloved object; a sweetheart. [Poetic] --Clarendon. [1913 Webster]

4.A woman filling the place, but without the rights, of a wife; a woman having an ongoing usually exclusive sexual relationship with a man, who may provide her with financial support in return; a concubine; a loose woman with whom one consorts habitually; as, both his wife and his mistress attended his funeral. --Spectator. [1913 Webster +PJC]

5.A title of courtesy formerly prefixed to the name of a woman, married or unmarried, but now superseded by the contracted forms, Mrs., for a married, and Miss, for an unmarried, woman. [1913 Webster]

Now Mistress Gilpin (careful soul).— Cowper.[1913 Webster]

6.A married woman; a wife. [Scot.] [1913 Webster]

Several of the neighboring mistresses had assembled to witness the event of this memorable evening.— Sir W. Scott.[1913 Webster]

7.The old name of the jack at bowls. --Beau. & Fl. [1913 Webster]

To be one's own mistress, to be exempt from control by another person. [1913 Webster]