My FP is and always was my brother. I literally can't imagine it being anyone else. He's two years older than me and even though I know it's objectively false, I feel like he's raised me. He matters more to me than anyone else and I literally cannot think of something I wouldn't do for him if he asked. Still, I've never told him he's my FP. I don't think he suspects, either- he doesn't know a lot of BPD. I prefer it this way, I know my thoughts about him tend to be obsessive and idealizing, and I don't want to taint our relationship with them. I'm happy just being in his presence and talking about memes or video games or shit.
Anyways, he's turned eighteen during the corona outbreak, and once it's over he's planning on moving out. I know it's wrong, but I've been praying that it doesn't blow over. I can't imagine my life without seeing him every day. Thinking about it makes me physically ill. I know he doesn't care about me half as much as I care about him and the thought of only seeing him at holidays and family gatherings for the rest of my life scares me. I've pretty much alternated my time since quarantine started breaking down in my room or trying to spend all my time with him. (I'm pretty sure he just thinks I'm worried for him because he has an essential job). 
I don't know what to do. He's leaving forever and I don't know how to stop him. I know he deserves to leave but I don't know why he wants to. I donxt want to get a new FP. I want him.
Side note: I don't know if this needs to be said but yes, I'm a minor. I've been diagnosed with "adolescent BPD." I know there's controversy regarding diagnosing underaged people, but if this isn't BPD (though I'm sure it is) all the things I'm feeling are still real and I genuinely need advice. Thanks.
Other side note: When I posted this in a general mental health subreddit, the majority of responses were some variation of sweet home Alabama/incest jokes. This isn't an incestuous relationship or fantasy. He's a platonic FP and I want actual advice instead of disgusting jokes.