Why can't I control myself around her?

There is this girl that I'm friends with, we've been friends for a few months now. I met her after I was already dating my girlfriend. My girlfriend is away in the Army. I like this friend of mine but I will not be that guy that cheats. I do flirt with this friend of mine, but I never let it go to far. But I starting to not be able to control my self around her. I can't help but touch her and flirt with her. I find myself opening up with her and having deep conversations with her. And I know she has feelings for me. What should I do?
Why can't I control myself around her?
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