Why won't my ex girlfriend let me go?

Hi. My ex broke up with me 3 1/2 years ago, after being together for 5 months. Since then I have tried to get back with her twice. Both times ended with me in tears. I said I didn't want be friends as it hurt to much. But she has always contacted me playing games and trying to be friends. I really love her and think about her every day.

Recently we started chatting on msn, her idea. She's in another country now. She said she was single and we were getting on really well talking 1-2 times a week for an hour or so. She was flirty and was giving me hope. Then it turns out she has had a boyfriend for 5 months,and lives with him. I was shocked she never told me for 5 months!. She knows I love her and don't want to be friends. So my question is why does she do this, she says she loves me as a friend but only hurts me. Why does she have a hard time letting me go? Does she just care about me, or likes being in control of me? I am going to tell her soon that I don't want no more contact, this will be the 3rd time I have asked her to leave me alone. I have been weak but she is very manipulative. Thanks for all comments.