Distance is difficult but in whichever way you usually communicate with her, tell her exactly what you said here. You love her, she is not worthless, she is important to you. Repeat often.

She should tell the school counselor/therapist and they will get help for her. If she tells her mom, the mom might not seek professional help for her. You can tell her kind words but she probably needs more serious help than that.

I agree about talking to a counselor at school.

Sadly, you could live in the same house with her & the results would likely be the same. She needs some professional help!!! I see a friend every day who has good days & bad days. When she’s having one of her worst days, NOTHING seems to make her feel any better & I know deep inside when I’m going to see some self-harm the very next day. I haven’t figured out a way to help her except to let her lead on what it is she feels the need to vent & try to understand as best I can.

Have you considered sending her an affirmation text every morning??? Something along the lines of “You are worthy. I love you & don’t what I would do if you weren’t in my life!!!” or something along that line, but in your OWN words.

Just repeat what you just said.

If she’s getting those terrible, negative messages, especially if it’s on a regular basis, no amount of counteracting them will truly help – she simply can’t believe good things about herself. It’s not a choice she’s making. I felt the same way for much of my life. I didn’t cut, but at age 15 I began taking insane risks and using drugs. She needs professional help asap.

Tell her that she is not the problem but her mother is and to get out or get help for herself.

I was a young adult when I stopped trying to have friends.
After having a few people close to me die, I believed relationships were nothing but pain waiting to happen.

Instead of trying to make a direct approach from far away, do something different. On Amazon and eBay, and lots of businesses with online shopping, there is an option to send a gift directly. So you don’t have to resend anything. Send her little gifts, anonymously. It doesn’t need to be expensive, and certainly not large, just something to let her know a secret friend is thinking of her. Maybe once a month.
I have gotten lots of keen jewelry very cheep on eBay. Oh, gosh, eBay-search jewelry lots. Some people box up dozens of pieces of jewelry and sell for ten bucks and up. It would be like getting a treasure chest with no special occasion.
If you can afford to, you can have flowers sent.
Use what you know about her to decide.