Can you go back to the doctor for more sleeping tablets? Even if just so that you can get through xmas.
Is there a duty line you can call in the holidays when you aren't seeing your therapist? If not then you can come here and ramble to us as much as you need to
You are definitely not alone with feeling how hard xmas is.
Being scared to lose people is normal when you care about them. Just try not to follow the thoughts for too long. They are just thoughts, a sign that you care, they are not real or likely to happen.
Going back to work is not for two more weeks. I would get through xmas first and then on a quieter day take a deep breath and ask yourself how you are doing. If it is still not good there is always the possibility to go to the doctor and ask for a sick note. But maybe you will feel a bit better after xmas is gone. So I would try not to think too much about it now. All the decisions can just wait.
Can you get an hour to yourself a day to do something you like that normally relaxes you.. like walking, reading or whatever your thing is or used to be. That could help with that break you need.
Just some thoughts, I hope at least one of them help, if now at least you know I am thinking about you. And none of us here is really alone because we are all in the same fight together.