My story is much like yours. Alcoholic ex husband who has alienated adult children. I truly understand your grief and merely offer some words that others have said to me that helped me.
Adult children these days have the idea that the parent caters to the kids. Meaning the parent does the giving, the child the receiving. Generations ago, parents felt their children had to earn the parents love and the children worked hard to earn it. Now, it is almost backwards, the parent has to earn the child's love. (I got this from Joshua Coleman's book, When Parents Hurt.)
Also an absentee father (like an alcoholic naturally is, even when home they are drinking not parenting), creates feelings of longing for that love in the child. So now that you are out of the picture, their father has to try harder to get their attention and they are loving having his attention and "love." It fills a void that his drinking created long ago. He wants them now and they'll take his love any way they can.
You write that you have tried to talk to your adult son about your feelings and your ex's behavior and the son doesn't want to hear it. You see, this is his father, it is natural he wants to love his father, and have a relationship with the father. Every child does. No child wants to have to choose between parents.
Lastly, you ask how to move forward, well you do it one step at a time. When we focus on the negative, our lives become more negative. When we focus on the positive, our lives become more positive. Just for today, find something good in your life, find something joyful, dance around the living room. Then just for tomorrow do it all again. Maybe its just that you cleaned the bathroom today. Do your best to let go of yesterday and forge ahead for tomorrow, do not let the pain and grief of yesterday rule your life today.
We are rooting for you here on WWU!