runnergirl250,
Here is the answer, and I promise that if you embrace it you will have a more fun senior season than you would doing really anything else...
Just accept the situation as it is. She is very insecure, but that is the teammate you have been given, and unfortunately at the high school level especially, we don't get to pick our teammates. People your age seem to think that they deserve all situations to be perfect. I am not slamming you here, but just do realize that for people a generation older than you, these questions you have wouldn't have even come up...we would have just dealt with it. If you just accept it as it, it is freeing. You don't have to like her, but you don't need to manipulate your season, your workouts, exactly to your liking either.
I agree with another poster who said it takes 2 to be competitive in workouts. Do your workouts and easy days at your pace, and if the coach is unhappy that you aren't racing easy days or workouts, then so be it.
Also, a well-placed "I think you would be faster on race day if you didn't run the easy days so hard" to the girl would be helpful. Lets her know that you aren't trying to beat her on those days and so her "win" of beating you is empty.
As for your mileage and whether a college coach would be interested in you, we don't have enough info to comment on that. if you would like some feedback on that, provide your weekly mileage and your PRs.
Good luck.