I think I remember your old thread, can't find it at the moment. I replied in there about her hobbies and friends?
I agree with you. It seems to be something that starts from a very young age and I think yours has only escalated in teenage years?
I think it sounds as though she has other struggles which is very normal in teenage years, and perhaps through online influences is thinking this is the reason for how she feels overall? this is why I was questioning hobbies and her life in general.
I do think a counsellor may help her explore her mind and thoughts and get to the bottom of it what she's truly struggling with.
I'm 27, so still very much remember teenage years, I really really struggled. Now I realise I was just so insecure and lost, I couldn't find myself, and acted out, I was mean to people, rude to teachers, I was mean to my parents, I was horrid! I don't even know that person I was. Mortified looking back....
you are her mother, you live with her, you know her best, as long as you still keep mind open to it incase it is the real deal, just hang in there. You sound like you've been very supportive so far