No-One gives good advice that I personally relate to with my 29DD.
replace old habits (which I ignored) and gently introduce, encourge new ways of coping. Anything self soothing my DD gained comfort, we gradually transformed DD’s bedroom into a calm haven, soft blankets she’d wrap herself in, cuddly cushions, mood lighting, new bedding – self soothing box with colouring books, pens, journal, hot chocolate, favourite perfume, creams, etc
My DD’s online journaled her journey from time to time, it’s been helpful for her to look back 3 years and see how far she has come and recognise how very ill she was. I can understand your concern about being safe online my DD’s on twitter and I had concerns to begin with – however she’s part of a group who share their BPD learning, support each other when they have a bad day, share DBT tools, skills, interesting articles (eg see The Mighty website) and helpful ways to manage. This has really helped her recovery working to destigmatising BPD/mental health, reaching out to help others, she’s not alone. Her greatest moment was receiving a tweet back from Blaise Aguirre, she read his book when she was diagnosed, he is her hero

What do you think your daughter may find helpful?