I DON'T KNOW. What should i do?

My girfriend got break up with me last year. She is really a over-smart girl and also phony. I love her very much , and i want to stay with him but, I know ,As boyfriend(lover) it's not possible. Because she is phony and she likes to make new boyfriend every year .As i'm not a smart and modern boy ,i think i'm not worthy for her. So as i want to stay with him(bcoz i love her very much) what can i do? Should i offer her "be my sister"!(sorry if a make any wrong in language bcoz i'm not good at english)