Hi
I’ve seen from your later post you’ve decided to turn down your family’s invitation…but I sense from your post that this is a burden…if you step daughter isn’t around over Christmas you may be resentful with your partner for missing the family Christmas (even if your partner insisted you still went to see family?), be careful not to let this drive a wedge between you..
What does your partner want / expect over Christmas, do they want you to still see your family? your family are important too…they might be the support network you need right now, I don’t think its selfish to want to spend time with them , recharge the batteries to give you strength for what you will face in the new year