Back in 1960s on December 18th my family lost my sister in a traffic accident along with my brother's fiance. It was a devastating time for us. We had to open presents on Christmas from her and it especially hit my father hard. I can't remember a Christmas after that where I couldn't find him taking a moment off in a corner by himself shedding a few tears. Many times I would join him and share his pain to make it more bearable for him.
I always thought that he did this quietly so the other family members could enjoy their Christmas without the painful memories that he felt. I was very young when my sister died so the pain isn't as sharp for me as it was for others. My father suffered this pain, and especially at Christmas, until he died in 1990. As I grew older I am thankful for sharing those quiet moments as it taught me just how traumatic this horrible accident was for him, my brother and many others. He did not dwell on his pain for long and allowed himself to enjoy his family and be grateful for having four other children and many grand children to enjoy and celebrate Christmas with.
I think he felt he had a duty to the family to be a willing part of the Christmas festivities. He wasn't going to short change the people in his life now by rescinding into grief over the loss of a daughter. This is just one of many reasons why I have such respect for him as a person and a father.
samlar, I hope you can compartmentalize your grief as my father did. Your grand children deserve to know you for who you are and not through a lens of grief and pain of loss. I don't think my father ever experienced a reduction in the pain he felt of losing a child but he was able to put it aside so his family could enjoy his participation in the celebration of Christmas. I hope you have, or can, do the same as you and your family deserve it.