You're looking for lines to get your girlfriend back. It's understandable considering the fact that you have absolutely no idea what you should be saying to get her love you again. Every time you think you know how to approach her, you try it and it ends with her being more upset than she already is. You feel that you can't win and it's causing you a great deal of anxiety. You love her, you need her back and you have to find a way to make that happen. It's possible if you fully understand what you need to say to her to get her to open back up to you again.
One of the best lines to get your girlfriend back is "I'm sorry." That sounds too easy, doesn't it? You may think it's unnecessary especially if she was the one who dumped you. It's crucial though regardless of who caused the break up or who decided to call it quits. By apologizing to her you're showing her that you are mature enough to own up to your own mistakes. Most people aren't especially right after a break up. They'd rather point fingers and blame the other person for the failed relationship. Show her that you can recognize your shortcomings and that you're learning from them. Apologize to her and make it as sincere as you possibly can.
Another of the things to say to your ex girlfriend is "I accept the break up." Right now you're likely fighting with everything you have to try and get her back. That hasn't been working for you and unless you change that approach, you'll continue to fail in your attempts to get her back. By accepting the break up, you're taking control of your own position in the relationship. No longer will she determine what happens for the two of you.
"Take care of yourself," seems like such an innocent and cordial thing to say to your ex. It speaks volumes though and can actually light a fire beneath her to want you back. By telling her to take care of yourself you're giving her the impression that the two of you won't be talking for some time. That will make her miss you and also make her see that you're ready to detach yourself from her. That feeling within a woman is often enough to make her want her guy back. There's a sense of panic that sets in when you realize that you are on the cusp of losing your ex forever.