How do I quell ual tension with the other woman?

I've been with my girlfriend for four years and I've never been unfaithful. I don't plan to be. Recently, she's made this friend who is like a walking aphrodisiac. She excites me and I get the impression this goes both ways. We bicker and get irritated when she comes over. It's like my body reacts involuntarily as if I'm 17 years old. I have concentrate no to show. I don't know what's going on here. Should I try to avoid her? She I discuss this with my girlfriend? Do I need a psychologist?