The Factory Girl actress, who starred alongside Knightley in The Edge of Love in 2007, has become a paparazzi target after a stream of controversial love affairs.
She has dated actors Jude Law and Rhys Ifans, and was at the centre of a scandal last year (08) after she started romancing married father-of-four Balthazar Getty.
Miller realises she has made mistakes, but insists she is still learning and doesn't possess the ability to make smart choices - unlike Knightley, who has managed to avoid such attention.
She says, "I haven't been as bright in some of the decisions I've made. Keira's just a more evolved person. I feel we live in the kind of culture now where you have to be very smart to navigate the right way, and I just don't have those smarts. I think with age and time it will change, but I can't obsess about it."