“I am dating a woman who has three children (8, 13, 15) from a marriage that ended long ago. She is determined to be a good friend to her ex-husband. This means the ex-husband is almost always in the picture and it seems like he is still part of the family. I am uncomfortable with this. It feels like they have never really divorced in some ways. Please help.” (Paraphrase of a much longer letter)

Get out of the middle!
No matter how much love you may develop for each other, as father of the children, her ex is going to be a part of the family equation. Any attempts, on your part, to interfere, or restrict his involvement, will come back to haunt only you. If the day comes that she wants to lessen her contact with him, I’d suggest it be at her initiation, not yours. I am fully aware that this may seem “off the wall” to many, but if you, the new man in the children’s lives, try to construct the dynamics according to your will, it will all begin, over time, to cave in on you. Leave any social re-organizing to the woman whom you are dating and offer her all the support she needs in facing her very tough task.