Because you've moved to someone else, but you're not over the girl. If she's with someone, let her be happy, I know its hard to watch her with someone else, but if she's happy thats all that matters, right? As far as your new girlfriend, I think it might be best to either work on the relationship and become close with her, or temporarily break it off to make sure you are actually interested in her and not just companionship.
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If idiots could fly, the internet would be a virtual airport.