Women please help.. ?

Why do women go back to the p*&^ks that hurt them.. And i don't mean just physical, it was emotional as well. She got rid of him a few weeks before we got together and i have never seen her as happy, smiling, laughing, virtually shining in every way as when we were together. She knew that she could trust me i don't get into something unless it feels right between us both and would never hurt anyone i care for, and this felt right. she promised never to hurt me, to always love her no matter what, she told me she loved me on our last night together. Then 2wks. later she ditches me and goes back to the friggin' twit that hurt her badly. Now she is having his child,(got pregnant a month after we split) yet she rings me a week ago and says that she loves me still (we have been split for over 3 months now), but she can't leave him because of the baby. I am in bits over this i love her like no one else i have ever known, together we were amazing, loved eachother, cared and trusted one another. (i know she left me) i was in pieces over her loss, but with the help of my friends i have, not got over her, but at least am able to get on with my life. I still miss her like crazy, love her unconditionally and would give my life to hold her just once more in my arms. There hasn't been a day gone by that i don't think of her. I can see her in every detail in my head, hear her voice and the words spoken between us, the feelings that she showed to me through her touch, her eyes when she looked at me, her small smile when we were together. Why did she push me away after admitting what she felt, and why tel me now about how she feels about me. She knows that no matter what has happened if she is willing to fight for us then i would gladly and happily take her and the baby, but as when we 1st split she is ignoring any attempt i make at contact. I'm just so tired, i don't sleep well anyway (2-3hrs. a night) now i get maybe half that. I can't get her or my feelings out of my head and its wearing me down.