The key thing to do here is to not get your ex back. You know that she is already happy with another person. Why are you going to try to win her back? Do you honestly believe that she will be happier with you? This is doubtful. A person can only be happy with the one that she loves.
If she does not love you anymore, she will find it hard to find the will to be happy. Will you also be happy knowing that someone that you love is not happy with you? What is the point of trying to get your ex back? Is it because you are truly in love or your ego is hurt?
First, I don’t think you can get your ex-girlfriend back if she is happy with her new boyfriend. Second, why would you want to get her back? She has obviously moved on and it would be better for you to also move on and find someone new. Whatever you may have done to make her leave you, learn from your mistakes for the future.
If you did nothing wrong and she still found someone new, that’s just one more reason you should not want her back. It’s also best to try and forget her completely. Don’t follow her on Facebook and don’t text her. Just pretend you never knew her.