That sounds really really hard. Do you think your ex is putting your kids up to this? Or are they doing this on their own? If your ex is not the ringleader, I would suggest you talk with your kids without involving her. Divorce is hard on kids, even adult children. They are hurt by what happened and holding it against you. They may be misinterpreting what happened in the break-up and have come to "blame" you. Or perhaps you do have some blame and they have not forgiven you. In my own marriage of 20 years, which is over, my XH did some things that he now wishes he hadn't, things that involved our kids and helped make them very wary about him, and untrusting. He has been trying very hard to put his relationship with them back together. At first he apologized to them, and then later came to realize that it was not going to take just one apology and then be done with that, but he was going to have to say he was sorry a number of times over a period of years and make good with his actions too. The kids need to hear how much he regrets some things he did and said from the past. I'm not saying he is constantly apologizing, but it is not just one time. His relationship with our kids is worth it.

That's a long story, but I think you should just go talk to your kids. Try not to view it as "confronting" them, but as having a heart-to-heart in service of making your relationship stronger. See what is up and why they are so stand offish. Before you go, look deep inside yourself and see if perhaps you know the reason. Any foreknowledge will only help you. Good luck to you.