It may be that she sees you as being too needy. This is not the time to project your insecurities and your need for reassurance. That's exactly what she wants a change if pace from. The more you grovel before her, the less desirable you become.

Forget about this ex not having any respect for you or your relationship with her. He has none and no reason why he should. All's fair in love and war. He figures, if he can win her back, then he should have her. He's going to go just as far with her as he can get. I don't say this to arouse your anxiety, but to help you see what's at play here.

You've had this girl on a pedastal that she didn't belong on because that's what you needed to believe. Now you're seeing that she can be quite a piece of work. Don't make your entire sanity depend on her being faithful. As long as you believe that you cannot survive rejection by a woman, you will not have her full respect.

When she gets back from this wedding trip, she needs to be the one who begs for your relationship to be resumed.