I feel for you @TheLostNights as I have an 8yo DS like this. He is the youngest of 3 and very much more “feely” than the other two. Their big feelings were much easier to deal with because they weren’t, well, quite as BIG as ds3s. When he has these big feels I do sit with him sometimes and just help him ride it out. Mainly because I am similar and my parents just let me get on with it in the corner which means that as an adult I have had to re-learn how to deal with my emotions as I never did when I was young.
He will often say Mum, I’m having a big feeling now- we will have a little chat about it, what it is/why he thinks he has it and I help him (I hope) put it in perspective as he is prone to overthinking too. Ends up with reassurance that feelings are good things and quite helpful, a cuddle and a happy smiley boy.
I would probably approach the long crying sessions with an acknowledgment of the fact she’s got some strong emotions going on and that you’re there to help her manage them so your days can continue and she can get to school/go to camp etc. We often have little hearts drawn on the palm of our hands in biro when I drop him off at school that we have “charged up” by holding hands during the morning so if he feels a bit of a wobble he can press the heart himself and get a shot of some mum love 😂
Some days he comes home with the heart worn away from pressing it so much and some days he forgets it’s even there the second he gets out the car, but it helps him in the moment when we’ve drawn them to feel reassured.
New emotions come out in strange ways- I remember when he first felt disappointed in himself (think he had been naughty at school and was a bit ashamed)- he was bloody miserable at home and a pain in the bum that evening at bedtime. Little chat about what he was feeling and how it was useful because it reminds him that being naughty doesn’t actually feel nice, and he decided that he’d try harder at school the next day. Cheered up no end and went to bed fine after that. But he was being a little sod at home because he didn’t know what the feeling was or what to do with it.
I agree with not pandering and she does need to know that you are there to help but not just allow her to go over and over the same thing for hours.l Perhaps if you make your attention productive and related to getting her to identify the worry then each time the feeling pops up you can remind her of how she dealt with it before and soon she won’t need you to remind her as she’ll be able to do it herself automatically. That’s what I’ve found anyway. Best of luck 