My boyfriend has another girlfriend. They are together for 1 year longer than me and him. I met her really early in our relationship, and I am mostly ok with her. But he really wants us to be close friends. And she wants it too. But I feel like I just... can't? I don't feel like we are even friends and you can't really make a friendship happen by force. We had sex, the three of us, and I had a very serious mental breakdown afterwards. I felt used and violated, jealous and guilty. They did not force me to do this, but I did it against my will because I wanted to please him. I wanted it to work with her. But now I am afraid of getting near her again. How do you make a relationship work if you can't be friends with your partner's partner?
There are two things going on here; I’ll address the easier one first.
Of course it’s fine if you don’t particularly want to be close with your partner’s partner! It’s okay for your boyfriend and his girlfriend to have a fantasy preference for everyone to be a cozy, friendly little polycule - but you aren’t obligated to meet those expectations. My partners sometimes date people I don’t really click with, and that’s totally fine. We just don’t hang out all three of us. If they are around, I’m polite, but I don’t need to spend all night at their hip, chatting it up like bffs. It’s disappointing when things don’t work out perfectly, but it’s also entirely survivable and fine. If your boyfriend keeps pressuring you to get closer to her, it’s okay to shrug that off and say “nothing against Merolda, I have no personal problems with her, but I’m just not interested in spending tons of time with her or trying to become close friends.” Then gently ask him to drop the issue.
The bigger issue here is why you don’t like being around her. It sounds like the threesome you had was pretty troubling for you, and you may even be having a trauma response to something your mind experienced as a sexual encounter “against your will.” It’s not her fault that being around her makes you feel afraid, but it’s also not fair to you to keep living with this fear. Please be honest with your boyfriend that you have a lot of painful and uncomfortable feelings around the threesome. They’re not his fault, but he can help you work through them. Take time to process that experience with yourself. What do you need in order to heal? What self-work do you need to do to prevent yourself from going along with the pressure of something like that again? Is there something your boyfriend needs to change to help you feel less pressured and more safe in this relationship? Don’t do this work with the goal of getting to be okay around her to make everyone happy - do this because you deserve healing, and because you can protect your future self from future problems like this.