What should I do? End it or change it?

First of all, if you are going to get involved with a pretty woman (all women are pretty to someone but some are pretty to more than others), you are going to have to accept the fact she is going to get hit on, and grow to the point where the physical is getting the right level of attention in your relationship. The fact she was willing to play the gullible, naive, part for a controlling guy, tells me she has issues regarding control. She may not feel comfortable about not having someone control, or manipulate her, since when someone is dong so, she knows where she stands.
The problem is that she was willing to let one guy control and manipulate her, while she was supposedly committed to another. This gives me two things to focus your attention on. One is that you were focused on how you were feeling about the situation, rather than talking it out with her and finding out what she was getting from him, that she wasn't getting from you. Maybe she saw the control/manipulation of texting, etc. as proof he cared about her.
Ask her to tell you how she feels when people talk with her in a manner you consider to be flirting. Listen to her and don't judge her. She has a right to her feelings, just as you have a right to yours. Respect her and be patient. Let her take her time and think it through. Then, talk it out and see where the source of that feeling, or need, is. Did her dad show her affection, or not? If so, did he stop when she started becoming a woman? If so, the flirting is filling the need for affection from a safe source which, as a boyfriend, you might not qualify as.
There are an infinite number of possibilities, so pray before, during and after, letting God show you the issues, and how He wants to deal with them. Don't just listen to her words, listen to and observe her non-verbal communications too. Listen for what she omits, brushes over, focuses on, etc., considering it prayerfully. Validate her feelings and concerns, even if you don't agree with them. It is not about you, it is about her.
After you have invested in her, share your feelings with her and let her know what she does that causes you to feel betrayed. Don't accuse her, affirm her and let her know that you see her as something wonderful and awesome, even if her behaviors cause you pain. Respect her and see if she will respect you in return. Treat her like the princess she is, but act like her prince.
It may be your turn to talk to the guys who are approaching her, sometimes, to let them know they are disrespecting you both by flirting with her. She is not the only one in the relationship and she is also not the man in the relationship. Work together to solve the problem and you will grow together. Make her work alone and you will grow apart.