Why can't I stop letting this girl get to me?

Sounds like a tough situation. If simply avoiding her and any situations where she will be present is not possible then I hate to say it but it may be time to expand your circle of friends. You sound rather young so I doubt your friends have the guts to stand up to her and tell her to take a hike so this may be the only option. People aren't going to cater to planning things making sure that either one of you isn't there to avoid issues because so and so doesn't like so and so. The absolute best thing to do is to avoid people liek this and just get them out of your life asap.