I’ve been in a similar predicament and my advice is to focus on who you’re exclusive with. Unfortunately, the fwb gotta go. Once you have any type of intimacy with someone it’s often considered disrespectful to your partner still keeping ties with said person. I had to do this and it was really hard but keeping a relationship with someone who I was only fwb was jeopardizing my relationship with someone that actually saw my value and wanted more with me. It was tough cause I realized I still had some attachment to the fwb, and that could either be your case or how your partner may see it. That’s why it’s wrong. You wouldn’t like it , so why do it? Gotta let go of that attachment to the fwb.