Can you change someones behavior?

I am 24, but my gf is 33. Very professional, hard working, educated individual. Her entire life she has only ever cheated on her gfs. Never once was faithful. We've been together for 3 yrs, had a lot of ups and downs. She claims she has never cheated, but does things like flirt, leave me out of just about everything, she has also tried staying friends with some tramp named sue who clearly wants to sleep with her and sends her naked pics. Long story short, I snapped, went off on her, and broke it down to her. She says I need to break it down more often bc she doesn't understand shes doing anything wrong as long as she isn't cheating, but to me she is still crossing a line. She said she won't be friends with Sue anymore and will start thinking about if the tables were turned how would she like it. I've broke things down for her before, and I feel like I shouldn't have to. Have you had to do this? does it really ever change? Do you really think you have to teach someone how to treat you? I just don't want to waste another 3 yrs. I love her more then anything in the world, but she really doesn't think about me or my feelings, or what may affect our relationship as long as she isn't "cheating" but it's still crossing a line, and i always have to tell her "okay with if I did this" for her to get it.. I don't know what to do. We work at the hospital together too (I am a nurse) We aren't fighting anymore, we've had good conversations about this, I am just afraid this is just an excuse and she will do something stupid again. I don't think you can TEACH someone things like this.. when it's crossing a line, and when it's not.