this might not be the advice you are looking for, but I feel that it is necessary to say it. When you love somebody, despite whatever flaws that person has, you can't hope but love them. I've gotten my heart broken many times, but I still love those fleeting moments. You need to sit down and ask yourself: Do I really love her? Despite her flirting, can I still love her?
If the answer is yes, be prepared to get kicked around. But methinks it's no. Cain, look for women that are both beautiful and sincere. It isn't that hard to find, but we often look in the wrong places. Take time to explore your surroundings, and eventually you'll find a girl who will love you for who you are. If Rebbecca likes to flirt with taken guys, even if you are going out with her, she's going to do it with someone else. And you don't want that, right? When a person shows you who they really are, believe it, then move on.