Oh lol76 I'm so sorry, and I totally get how you are feeling. Boxing Day was the only day we were going to see anyone - my husband's sister & her family were going to come from London. Now obviously they can't. It is really disappointing.
I think the way you are feeling means that no words from me are going to help. I am not in a place of depression at the moment (luckily) so I can easily try to look on the positive side (less pressure, more coziness, can do whatever I want, don't have to worry about cooking for anyone). Very difficult to look on the positives when you're feeling as you are feeling. I know, I've been there.
So I think the best advice I can give you is to be really really really kind and gentle to yourself. Don't expect too much of yourself, don't expect to feel great and Christmassy. Treat yourself like you'd treat a friend who wasn't very well. Give yourself a warm bath. Wrap in in your comfiest clothes. Go out for a little walk every day but don't expect too much, just walk round the block. Watch a movie that you like. Try to eat 3 meals a day even when you don't feel like it. Just getting up and carrying on and getting through a day is a big achievement and you should feel proud of yourself at the end of each day for getting through it.
The government are handling the whole situation really really badly. I think a lot of people are feeling very disappointed and just like we've had enough of it all. But hang in there. The seasons will change. Spring will be here again soon. Nothing lasts forever.