Hey Ino,
firstly expressing these thoughts is the best thing you can do, to actually become more confident and true to yourself.
Now, I noticed that you call that girl "friend" and this shows more than you thing. Believe it or not, in my opinion, you actually care about that girl and not only as a rival, you care about her as a person and do you know why? Because somehow you see a part of yourself in her. You recognize that there are some things that you have in common (for example rivalry at clothes shows that you are both clothe-lovers and other examples of these little things that express our true self, I'm sure you understand what I mean).
So, I think that the question you must ask yourself is why have you chosen this girl to be your rival (same goes for her too) when you could chose her as a friend? Because we chose our rivals or they chose us, exactly the way it happens with our friends.
For the theoretical part, my basic principle is not see things as good or bad. Not all things are black and white, so I don't think that there's a "right" answer to your question. For example, L. D. Vinci's rivalry to everyone else gave us the Mona Lisa, but Achilles' rivalry to the Greek Gods, led to a dead Hector, to a dead Patroclus and to the death of himself. See?
Hope I helped you a little! And bravo for your grades!