Op I am going through the challenging teen years and I really feel for you
. A lot of us are feeling down ATM. I think the pandemic has been tough as we have been doing all the hard bits without having the relief of the fun bits in between! Don't be too hard on yourself. You have been parenting through a global pandemic dammit, so pat yourself on the back and give yourself some credit! 
I agree with pp that some DC take longer to develop confidence than others, and starting secondary can be a very tricky time for girls and friendship issues.
How confident are you in yourself? Children tend to do what we do and not as we say. Try and step back slightly from the minutiae of their friendships at school. Obviously it's very hard if your daughter is crying and the line between "being sympathetic" and "being encouraging" is very difficult to get right. It's not our job to remove problems from their lives though; but to teach them how to negotiate the bumps in the road.
Were you positive with your DD last night? Did you teach her that her friend being away was an opportunity to become friends with other people? Obviously while acknowledging that the start of secondary school can be difficult for everyone and that it's normal to feel scared and upset sometimes. Everyone does! And that school is a place where we have good and bad days and that's ok.
Do your DC have any responsibilities? Maybe a pet like a guinea pig would help them feel responsible for something and that would help their confidence? (But be careful not to add to your own burdens.)
Do you have fun together as a family? Board game night? Bicycle rides? Maybe set yourself a Conqueror challenge as a family and support each other? www.theconqueror.events/
How much time do your DC spend on line? The on-line world can be hugely undermining, especially for young girls, as they are basically comparing themselves to hundreds of You Tubers and Bloggers and Influencers and it's very unhealthy mentally.
Maybe your DD is an introvert who enjoys being at home? Is she creative?
Maybe this could be the perfect time to facilitate her painting, drawing, sewing, baking?
And do you have enough support and are you getting out enough op? Are you doing something you really love and enjoy that feeds your souls once a week? If not, please make it happen! You need the respite and you will be modelling good self care and happiness to your DC.
Could you go away for a long weekend to recharge and leave the DC in the care of someone else? Just to recharge?
Sorry for the random thoughts above! They may be totally off the mark but they are things that have helped me in the past when everything has got too much. Good luck 