Her last message to me said “I’m so so sorry to send you these messages but you’re the only person I can talk to”
People with mental health problems can be really manipulative - whether it's a part of the MH illness or their personality is probably not for us to internet diagnose, but a few things you have said about how she is with you makes me think she is being really very manipulative towards you.
All of this "the professionals don't care, they don't listen like you do, nobody cares about me as much as you do" is really piling on pressure on you to continue to fulfil the role that she has decided that you play - to the detriment of your own needs, your relationships and your happiness. That might be the illness talking, but it is also undeniably selfish and unfair on you.
I understand why you want to continue the friendship, and I support that, but you do have to learn how to stand up for yourself, both in this relationship and I would guess elsewhere in your life. Plus if you don't mind me being blunt for a moment - being there for her as much as you are hasn't improved her situation at all, has it? Because of the aforementioned she has to want to get better, and she has to engage with professionals. You're doing both yourself and her a disservice to continue to be so available for her wallowing.
Well done on the boundaries around animal cruelty videos. You've proven to yourself that you have what it takes to create healthy boundaries, and that's huge! Now to apply more of them.
I have a friend who can get incredibly needy. She's always going through some kind of drama or other and even though most don't seem to be created by her, they do have a funny habit of happening to her, time and time again. I thought she would lean on me less when she got a fiancee, but he seemed to get the best of her and I got only the worst, and that combined with one particularly bonkers week was when I needed to have a break from her and her drama. I didn't do the mature thing and tell her immediately, but I kept phone calls brief "Hi friend yes I can talk but only for 5 minutes" followed up by "So sorry, as I said I have to go, speak soon!" and I let many go to voicemail. Eventually I got the courage to tell her that all of the drama was having a negative impact on me, and I needed to have a break from intensive contact with her, and she respected that. It actually took us months to get back on an even keel, but our friendship was strong enough to handle negotiating the choppy waters and we were honest (and kind) with each other throughout it.
If it's a friendship with a solid foundation, you'll be able to navigate this together. I've had friendships which weren't so well founded break down, and that's because they didn't want to be able to deal with what they were responsible for, and also because I wasn't too good at communicating my needs. If you want this friendship to survive, you need to 1) honour your own needs and 2) communicate them with kindness.
"Friend, I have a need for rest and time with my family. If you have a need to talk to somebody this weekend, will you ring Samaritans instead of me please? Let's chat on Tuesday if you're free then."
"Friend, lately I've been feeling like I'm struggling with supporting you. I want to protect our friendship and I also need to protect my own mental health. I want you to be safe, happy and supported - can we please negotiate a situation where you seek some of that support elsewhere?"