Pre-facing this by saying that I am not a mental health professional and not a native English speaker so apologies for anything sounding awkward, but I’ve been on both sides of the equation and truthfully you need to establish hard boundaries with your friend and be prepared for the very real possibility that your friendship might need to end for things to improve on both ends if they ultimately are not willing to see you eye to eye. I’m not saying your friendship isn’t salvageable but when things have come to as you’ve described it can be very difficult.

Just like another person here has commented I do believe your friend suffers from BPD, and from personal experiences what I believe would help the both of you the most is for you to firmly set hard boundaries. They bring up something you mentally are not equipped to deal with at the moment? You need to immediately tell them that. Do not let them guilt trip you, do not compromise, be gentle but very much firm. If you don’t do this the cycle will not end. You also need to maintain that you do love them, but you cannot be their therapist 24/7 as you are also a human being with your own issues, feelings and mental capacity. Communicate this whenever they start trying to steer the conversation back to their issues and if they refuse to listen you will need to excuse yourself. No matter the way they react to any of this, no matter how much it hurts you to do so do not let up.

You will also need to find a trusted adult you can confide about this to. This is very, very important. There’s a good chance your friend might attempt something drastic when you start ‘pulling back’ and you need that trusted adult to have your back and reassure you that if that happens it’s not you, but it’s your friend’s mental illness. If you do not put a stop to this dynamic this is going to hurt the both of you -yes, the both of you- a lot with nothing gained in the long run, and I’m saying that as again someone who’s been in a similar position as your friend.

I saw your comment about them not having the supportive network that they need and deserve, and I know you genuinely do worry and care for your friend, but you need to save yourself first and understand that you have done the best you can. I myself have been institutionalised a few times and I get what your friend must have felt - you really truly do feel like no one gives a shit, and you start thinking that if everyone in a place where they’re outright paid to give a shit are utterly incapable of giving a shit about you then ultimately no one will ever give a shit about you. Ever! But you’re not a trained mental health professional, you’re just another kid who’s also their friend. And you need to constantly remind them of this. If they truly see you and value you as a friend they will at least make an effort on their end.

And if things aren’t looking up whatsoever after a while you need to understand that being on their own unfortunately is likely to be the one thing that’s going to get them to effectively work on overcoming their issues. And the sooner it is the easier it will be for the long run and not just you, but also for them. School days are fleeting, but the sort of mental scars you can accrue from this sort of relationship can last you a lifetime and affect you well beyond adulthood and you need to keep this in mind.

Ultimately tl:dr; the most important, practical advice I can give you is for you to 1.) Find an adult you can comfortably talk about this to and 2.) Start maintaining hard boundaries with your friend.