Why is my estranged wife doing this to me?

As a rather shy student I met my wife who was older than me, she was the only woman who had ever shown any interest in me and I was so grateful for her attention I asked her to marry me after only a few months for courtship. I did love her but over time she became more dissatisfied with me in particular my appearance for I am not handsome by any stretch of the imagination, also my work is theoretical and other than walking I did not get much exercise and so had become very "soft" in appearance by the age of 30. I longed to become a father and was delighted when my wife fell pregnant with our child but my son was only 1 year old when she walked out on us she said I disgusted her leaving me devastated and alone to raise our son. It took me a long time to get over it although now I see that that both of us were just making do with 2nd best in each other. Earlier this year after being alone for half a decade I met another woman she is 32 warm, beautiful, creative and intelligent. We were introduced by a rather wise friend who saw we would be perfect together and our first time in bed together was a revelation of both love and deep pleasure I had not known possible. She is my soul mate, my son adores her and I want only to make her my wife doing it this time for all the right reasons. I have very limitied contact with my wife who only sees our son a few times a year I spoke to her about arrangeing a divorce so that I can be free to marry again I thought she would be glad to be free but now she knows I am in love with someone else she says she wants me back, and thinks we should be together for our son's sake and so on. I no longer have any love left for her and will not take her back but she is now making things difficult for me and my new partner, why is she doing this to me now that it is too late for us?