Hello, my girlfriend has always had rough times but more recently with pandemic stuff she stayed in the house a majority of the year, to the point of getting so depressed that she admitted herself to the hospital. We've discussed getting help many times before but she never truly expressed a want for it, I've also always been a mind over matter guy. I believed we could do it without medicine.
With her visit she is now in the process of being re-diagnosed with BPD, and since then I've been doing as much research as I could to try and figure it out, realizing that maybe we cant do it without medicine, and we sure cant do it alone.
She moved her cousin into our apartment shortly after being released and ever since they've been inseparable, as in every single day and night the last 4 months. 
We don't have a friendship anymore, I feel like the time right after a traumatic hospital stay (her first time getting help) and starting medication is a very important time for healing but she refuses, saying our home doesnt feel right for her because of the dark times before the admittance.
I'm afraid shes falling into a crowd that could potentially turn bad, shes absorbed her cousins personality and flipped her whole life around all without talking to me about it.
I see all these descriptions of stereotypical BPD toxic relationships and I feel we fit the bill pretty well. I'm pretty narcissistic and I have my bad habits but I do actively work on them, this is my first relationship and I want it to work out but we cant seem to have a single conversation without it turning poisonous.
I instigate kinda often and I try so hard not to but I dont really have any coping methods, she will be full manic, screaming and threatening me, then act like nothing happened and refuse to nurture the pain I have from it
I've asked to talk like adults about splitting up and she immediately cries and gets mean, saying shes in it for the long run and I can essentially fuck off we arent breaking up. She has spent almost $3,000 in 3 weeks!! that I worked extremely hard to save and we havent paid rent in 2 months because she swears we're getting rental assistance, that I wasnt part of the process of applying for. And this is after I decided to take full control of finances and told her do not touch my debit card. It's only tuesday, I was paid friday and I wont have gas money to get to work next week because my account has been dried. From now on I will be deactivating my card until I'm ready to use it but I'm so stressed out altogether all I feel I can do is post on reddit for perspective