This is my first post here. I'll be thankful for responses and will try to reciprocate.

Several years ago, I had a coworker I became very good friend with. Her behavior however deteriorated and I started getting very stressed out by her. She would basically drop me every time she had a boyfriend, then want to be super close when she had nothing better to do, never came to work but got away with it because she was really chummy with the boss, etc. She was diagnosed as borderline bipolar at some point. Eventually she went back to Europe. Our friendship was on very thin ice towards the end of her stay, but I've been able to maintain it thanks to her being so far away and over the years she's become more reasonable.

She's back in my office now for a few months, finishing a project. I wasn't happy about it, but it's up to the boss so I've stayed quiet and have been friendly with her, probably too friendly. I've been really scared that she'll want to stay in the US and try to get a job at the company again, even though she's been saying for a while now that she's sure she wants to stay in Europe. Predictably, she spotted a job opening in our company and she immediately decided that she wants it, which has created huge anxiety for me.

This friend always copies me. I know that sounds petty. I've wrestled with it tremendously, but for years, it feels like anything I do, she has to do. I've been working remotely for a while, but recently decided to move back. In the process, I bought a condo. She said to me basically, "Oh, I see you've decided to move back and buy a condo, and I think it just makes a lot of sense and maybe that's what I should do too." I know that that isn't a big deal, I don't own the city or the act of buying a condo, but it's a pattern. Very recently, I also got a promotion, which was sort of a big deal. The job she's looking at is basically the exact same type of position. She's also decided that she wants to go into big data and machine-learning, which is my field, and goes on about how she's learning R and it's so "interesting." I remember how it was years ago, that first time she was around. When she found out I was taking piano lessons, she immediately said, "Oh, I always wanted to learn the piano." When she learned I was knitting, she wanted me to teach her. When I happened to tell her about some Jewish holiday I was celebrating and the meaning behind it, she was like, "Wow, that makes so much sense, maybe I should convert..."

I would have stopped being friends with this person, but my boss has a thing for her and I just knew that I'd never be free of her while I work in this office. However, I'm just extremely depressed and agitated thinking that I'm going to have to deal with this all over again. What's more, and this is the worst part, she and I often have a great time together. So I admit fault, that I've sent the wrong signals. What if she gets the job and decides to come back? I honestly don't understand myself and why I just can't seem to think about anything else, but this person is just an incredibly toxic presence. All day long, I've just been caught in a negative spiral, dwelling on all the things she's done over the years and how I literally can't do a single thing without her deciding that she wants it for herself. Does anyone have experience with this sort of friendship? In the end I think I just have to stop being friends with her, but damn it, it's really hard if she's coming back to the office! I'm really afraid of how she'll treat me, should I give her the cold shoulder.