Fate is my mistress
I heard the phrase by Cesare Borgia in the TV series Borgia - Faith and Fear. I think it means that the speaker is in control of his destiny or that he needn't fear any outcomes of his actions for his fate is always on his side. However, I can't seem to find any definitions on the Goggle search for this usage.
And apologies as I can't seem to call the entire dialogue. But Cesare says this after Cardinal Della Rovere asks him as to how he was so confident of the plans he was laying in action.