I would love to put in my input but I feel since im only 16 and been in a realtionship now for a year with my girl it wouldn't help much. I do relate to a lot of the problems you are having though. I've had the same problems and I just kept telling her that I want her to stop talking to a certain guy. Then out of no where she came to me and told me she was through with talking to other guys cause she only wants to be with me and that no other guy means anything to her and she doesn't want to lose me.
I'm a pretty big kid at my highschool (6'5) and the other kid was a little scrawny guy about 5'9. He thinks he is the biggest bada** at the school and can say and do what ever he wants. He use to text my girl everyday and she would text back always. That use to piss me off so much I just wouldn't talk to her cause, well not to be cocky or anything but all the girls at my school love me, so I would talk to them. This went on for about the first month in our realtionship so I sat down and talked with her and told her that if she doesn't change the way she goes about talking to guys then I will simpily just leave her. I told her were in highschool I don't want to be dealing with this during basketball and being pissed off at you. The next day is when she came up to me and said she'd stop.
We've now been together for almost a year.(about 17 more days) I've learned to just ignore her talking to other guys. We went on a couple single day breaks (which are pointless) because I got mad and said the wrong thing. Always think before you speak. We haven't had any problems in the last, well, 5 months and ever since I started to ignore her talking to other guys she suddenly just stopped completely. So you could easily just try and ignore it for a week or so and she how that goes.
Another thing is try and spend as much time with her as you can. I know that the more time you two have together while you aren't fighting the happier you two will be. If everytime you spend time with her she is on her phone with the guy take her out to do something where she can't carry her phone with her and text the whole time. Show her how much fun she can have with you while not talking to the other guy or even thinking about the other guy. For me everyday me and my girl walk to all our classes together. As soon as I get done with practice and she is done with soccer we hang out every night.
So all sumed up just try and ignore it and spend as much time with her as you can. I know it helped out my relationship but yet im just only 16.
