My boyfriend got back with his ex, who I think is very bad for him

My boyfriend is poly, he has gotten a new boyfriend recently and he’s dated this boy in the past, he’s broken my boy so many times and apparently he’s gone to relationship counseling to fix himself. My boyfriend insists he has changed after three months of the counseling, they got back together a few days ago. He asked me if I was comfortable with him dating two other people I said this was fine but I didn’t ask who they were and finding out he’s with this guy again upsets me.

There first issue here is that your boyfriend seems to be lying by omission, saying “are you okay with me dating someone?” knowing that your answer will be “yes” in the general case, and neglecting to mention that it was this specific person. You need to make sure there’s an open line of communication between the two of you about your relationship, new partners, and concerns either of you have. If this is a pattern, you need to figure out whether he’s being evasive because he feels unsafe being honest with you, or just to avoid an unpleasant conversation, or what.

You can’t change your boyfriend’s mind about dating this person - but you can control how you respond to it. You can decide that this person is just not someone you want to be polyamorously connected to, and choose to leave the relationship if it means having him as a metamour. You could decide that you’re not willing to discuss this person with your boyfriend, and that includes advice, comfort, venting, anything. You could decide that you don’t want to be around this person. But all of those are your choices, based on the information you have: that your boyfriend is going to date this person, despite your objections.

Consider also checking your language: you say he has “broken” “your” boy, and that might signal some extremity in your perspective. Your boyfriend has not been “broken” - it’s pretty difficult to actually “break” a person - he may have been hurt, disappointed, angry, frustrated, sad, etc. Using clear, specific terms for feelings helps us address them better. And he is not “your boy” - he is his own person, and his relationships with other people are mostly his business. I don’t doubt you that this person is emotionally dangerous, but don’t make it worse with language that frames your reaction in extreme terms that make things harder, not easier, to address and resolve.

Instead of talking about what this person has “done to your boyfriend,” focus on the impact it’s having on your - “seeing my boyfriend so hurt makes me miserable, and I don’t want to be in this situation, having this person as a metamour is not right for me.” That’s a much better framing, because it focuses on what you can control in your responses.