Hi there. So sorry you feel this way. I think the constant bombardment of happy family adverts (these are actors who have never met) and social media, 'look at us - we're having a great time' (you never know what's going on behind closed doors, take it with a pinch of salt) - I do it myself. All this illusion is at its worst at this time of year. So first off, pay no attention to it, if you can - it just makes you feel worse.
My next bit of advice is what I gave myself when I gave up smoking or if I'm on the verge of a panic attack. I'm just going to get through today - literally, just today. You can watch the hours tick by, that's fine, but this time WILL pass, Christmas will end, a new year begin very soon and you can think about the details then.
The summer holiday adverts have started, it's not too far away.
Focus on your kids, when you start worrying, interact with them, really plug into what they're doing and saying. Read them a book, take them for a drive to get out of the house, whose got the best lights. Get on the floor and play with them. Take them to visit someone you feel happy with. Maybe the cinema when they break up or a walk when the weather is not so shitty.
This immediate, awful time will pass. The awful time of you being in an unhappy marriage will pass too, when you take action - but don't think about that now, just get through today.
I hope this helps. Wishing you all the best op. It's a crap time if year for many people, for all kinds of reasons, you are not alone. I lost my daughter to cancer, I hate Christmas with a passion. I'm counting down the days for it to end too. And it does end. This will pass.
Big hugs op, wishing you a rosy future, down the line. 💐💐💐