Even if our partners ask us to send lawyers, guns, and money, we need to stay on our own path and offer them an empathetic ear and a solid base of loving AND SILENT support.
If your kids are old enough, talk to them about how they would like to do Christmas within the schedule you and the other parent have determined. Over time, it does get better. But while things are fresh and hard, make sure you take care of yourself first.
When you begin building a relationship, part of what you are envisioning is how the two of you will progress into a full-blown, loving, couple. All the dreams of what life will be like with this person are there for your enjoyment.
I may not reach serenity with my ex-wife and her new husband ever. That's okay. The serenity is within me. I am doing, have done, and will do the best I can to support my kids and keep my relationship to them above the fray my ex continues to keep seething around us.