How can I get over her?

First, if you feel that this is just silly high school drama stuff, please just move on, don’t comment, as this is a very sensitive topic for me.

Last year I met a girl. We did everything together, went out on dates, first with friends, then one on one. From about the second day I knew her, I knew she had a boyfriend and I decided to ignore it. Over the course of about 10 months I fell desperately in love with her. We did everything together and I lived for the next moment I’d spend with her. It was easy to ignore the fact that she had a boyfriend; we’re in college and her boyfriend was “back home” [4 hours away.] While we were together, people thought we were dating. We drove across the state (4 hours) to a concert, we went to the beach together. We even shared a bed a couple times. When she had some really rough times, she called me. And I, her.

Over the summer she went back home and I didn’t get to see her much. Due to monetary constraints she didn’t make it back here. She had to drop out and attend school back home. Last semester I forced myself to move on, even had a girlfriend for about a month. But I just couldn’t love her like I loved Emily.

Emily came up to visit last weekend. We went out to dinner with her friends. As usual she was all over me, picked up right where we ended, we pretended we were dating.
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But I can’t keep doing this. I have to get over her, because she’s made it clear she’ll not leave her boyfriend for me. And right now, if she comes back up here, I’m gonna go right back to her, like nothing ever changed. I’ve decided I can’t do this anymore. I want to move on but my fear right now, is that if I move on, start a relationship with someone, if Emily comes back, I will end up going to Emily.

How can I completely get over her, to the point where I can tell her no when she comes back, regardless of whether or not I’m dating someone?