Sounds like you have some very valid reasons for feeling overwhelmed and sad, OP. I've been in the same place before. It's a normal response to life difficulties (such as being responsible for your teens who are both struggling, and not having much support). The run up to Christmas is a lethal mix of nostalgia and stress/expectations so it's not surprising you're even more sad today. In my case, I tried to wait it out but it got too bad and an antidepressant helped me until my life circumstances were a little easier and then I phased off it.
Does it help to acknowledge things are tough, and the way you feel is a reasonable response? Rather than seeing your feelings as one more problem to be solved? Once you accept that you can figure out how best to tackle life so that you don't dip any further, and how can you bring yourself up just a little bit.
I'd echo PP's with simplifying everything right down to survival mode, and adding anything you can think of that will bring you joy. Forget doing all the things you might have been able to do in previous years when things were easier. Look after yourself. You're the centre of your family's life and you need to be kind to yourself.
I'd also consider talking to your GP about whether a talking therapy or an antidepressant could help you.
For what it's worth, you sound like a thoughtful person and a lovely mum. You're doing an incredibly difficult job successfully since your DD is at home rather than in hospital. One step at a time. I hope you enjoy your walk, and hope that Christmas is better than you expect.