She may seem old because you have younger children but really she is very much still a child. She will be mess up - whether with money, relationships or whatever - she is still very much in the infancy stage of adulthood and has limited experience of managing. I think you need to let go of your expectations that she manage things seamlessly: she won't.
Obviously she is feeling unhappy, that is crystal clear from your description of her behaviour. Undoubtedly you two are in a pattern of triggering each other and it really is up to you, as her mum, to try to change this. If you keep doing what you're doing, you will get the same results. So something has to change.
I think you need to practise taking time before you answer her calls/texts/demands. Don't get into conflict. Always be civil, and try to be loving. Try to be consistent with your contact... check in on her without asking anything of her. Don't dig your heels in about her missing family occasions. Anything at all to reduce drama and to encourage positive contact.
She is only a teenager for a short period of time (even though it can feel very long!) and there is every chance she will emerge, butterfly-like, as a responsible and more considerate young woman in a few years.
I don't suggest that it is easy because I know it is not, but hold onto hope and remember she is as much your daughter today as she was the day she was born.