I'm going to tell you some things that are probably not going to be very nice to read, so if you are a sensitive person you probably don't want to read any further. However I am not trying to be hurtful, because sometimes a person needs to have their feelings spared and hear some real honest truths. "Oh man, that's terrible, I'm really sorry" may sound nice but isn't going to help you in this situation.
You need to let this woman go. She doesn't want you anymore. "Separated", "remain close as friends", "less than perfect relationship", "vagabond friends", "stop checking up on me" all make that plain as day to an outsider. I understand love and mental illness just fine. I understand those concepts enough to tell you that first and foremost, you need to love yourself, and don't allow anyone else on this planet to put you through situations that negatively affect your own mental health. Which is what is happening right now. I've dealt with people like her, if you give her the option of treatment or divorce, she'll go through the motions of treatment just enough to keep you off her back, and keep you around to fuss over and worry about her, work, pay the bills, and provide a home for her. Then she'll quit, and it will be the therapist's fault, or some excuse or another, and the cycle continues.
She is sick. If it was cancer I wouldn't do simply divorce her because of the way the illness was affecting her, why would I do it with a mental illness?
Because BPD is a very difficult illness to treat even with someone who realizes that they have this illness and want treatment, and running around in the woods off her meds and taking illegal drugs is not going to improve her condition. Loving her and worrying about her is not going to cure her. You can't save everybody, and not everybody wants to be saved. If the 2 years of self destructive lifestyle she is living now is what she wants, let her have it, move on with your life, and find someone who will reciprocate your feelings and treat you like you deserve to be treated. Right now you're making excuses for her and enabling her, and that's not good for either of you. You said she's been off her meds for 2 years, how many more years of your own happiness are you going to sacrifice? What about YOUR dreams, happiness, and goals? If you were told that today was your last day, how would you feel about that? You can never, ever make up for lost time.
I understand someone being a part of your life long term, and the security/familiarity of that, but you're not in a healthy situation. Sometimes it's better to be alone for a while than to be in something unhealthy. I'm sure you're probably think "oh, but you don't understand/don't know her like I do" etc, but believe me I have a lot of life experience, I have a reason or experience for everything I'm telling you.
I'm sorry for being blunt, but you really need to wake up tomorrow, brush your teeth, wash your face, and look in the mirror and tell yourself that you don't deserve this shit and believe it. And take control of the situation rather than letting it control you. I dealt with abuse as a child, and I had that day just like I described. The feeling of empowerment was overwhelming, and nobody ever fucked with me ever again. You can do the same. I do wish you the best because one of my core beliefs is that everyone has the right be happy.