Can your SO be "stolen?"

There was one girl who I vowed to hate for the rest of my life. It was the girl who stole my ex-boyfriend. She was a new friend he had made through an organization on campus while we were in college. When I first met her she seemed nice. Then as time went on I started to suspect something between them. I noticed they were becoming better friends, and the way she would ask him for favors, post messages on his Facebook, chat online, and interact with him made me feel uncomfortable. I approached him about it, but he reassured me that they were just friends and that he could never date someone like her. He even brought us both to lunch once because he wanted us to get along. A little while later we broke up, and about a month or 2 afterwards, he told me he was already seeing someone. As soon as he said that sentence, I already knew who it was.

I was devastated. The timeline I put together made me certain that they liked each other while I was with him, and that the day he broke up with me is probably the day they decided they wanted to be together, maybe even possibly cheated (it's not confirmed... but I don't want to fill up this whole post with why I'm pretty sure that's what happened. I'll just say that events transpired that led me to believe this). I hated him for betraying me. But I truly hated this girl. This heartless, monstrous bitch, who plotted, schemed, and stole her way into his heart.

Since then, I've had a long time to reflect on our relationship and what went wrong. We were having problems during this time. We almost broke up, took a break, and stayed together. I think around the time he started liking her, he was already gone from the relationship. Whether or not this girl was in the picture, we would have broken up because we were just not right for each other. And if he was already "gone" from me (mentally, at least), that means he could not have been "stolen." Nevertheless, while I have forgiven him, it is hard for me to forgive her. I still can't look at pictures of her without feelings of disgust and hatred.

Do you think an SO can be stolen or is he/she already gone? Have you ever stolen another person's SO?

**Edit: The datingish blog featured my post! http://www.datingish.com/715286358/can-your-so-be-stolen/