Don't have feelings for her anymore.

Hello, me and my girlfriend are together 5 months now, but after a huge fight I feel like we don't belong together anymore. She wants to stay with me because she loves me but I don't feel as I used to. Not only that, the last 2 weeks I am having a huge crush on her best friend that we are all 3 together in the same class and we always go out together after school and have fun. I don't think there's a possibility that she might like me back, but I don't want something to happen with her because I could never do this to my girlfriend, even if we break-up. I am so confused and I don't know what to do, anyone got any advice?
Don't have feelings for her anymore.
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