Rach, you have been given a lot of advice that you probably already know the answers to, like you can well walk the walk but not talk the talk,I don't know you no more than I know anyone else on this site, but I think from reading your post, your 'children' are now adults and relationships change, you come home from a demanding job I am guessing with no one to share it with, I don't know what you see in your work but maybe it's dragging you down too. I don't believe everything you touch turns to a pile of asterix', you're obviously helping someone, if you could hold onto that thought and think who went home today all the better for having being in touch with you.
No problem is so bad that it can't be solved, you have family, maybe some family mediation and someone else telling your son and daughter that you are not coping with the burden of feeling so isolated - how they are treating you, may make them sit up and think, there's loads of reasons to stay alive, young people today say YOLO, you only live once, but that's untrue, you live everyday, die once, it's final, so every day you live is a chance to change what's so wrong within, if you can't afford counselling from NHS - there is a great site (I know the Samaritans are there and other listening groups) but I know Moodgym - cbt it's an Australian site, it's very good and I know it was recommended to a friend of mine - she found it so useful when she couldn't afford private counselling, things always seem worst at night too, try have a warm drink, and settle yourself down to sleep if you can, and if you can't, why not look at least at the link I provide and maybe it may be just too obvious the answers and exercises for you, but at least it will distract you for a while
I'm sorry you're feeling so low - keep talking xx people do care
https://moodgym.com.au/