Lana Del Rey - Sad Girl Lyrics Meaning

"being a mistress on the side ...it might not appeal to fools like you" means that she has been the side girl for all these years and all the men she has been with are so blindminded that they don't see that she truly needs them or him in her life,they don't know how much she craves the love of another being. " sad girl " implying shes sad about all the other mistresses and shes just another one on the side ,shes trying to show him how sad you can make people... from them fining out they are just another side girl...which in my opinion is heartbreaking