My boyfriend is poly but I’m not and he really wants to bring my best friend into our relationship (who he has had a previous relationship with) and keeps suggesting it to me, I’m trying to make him understand that I can’t force myself to do that. It’s making me so upset and I don’t know what to do

Remember that “no” is a complete sentence. If you’ve been very clear with your boyfriend that you do not want to include this person in your relationship, and he continues to push, you need to lay down that boundary: “I’ve told you that I will not do that, and it makes me very upset when you refuse to take no for an answer.” 

At that point, if he still refuses to “understand,” that’s on him. You may need to reconsider whether you can be in a healthy, happy relationship with someone who cannot accept your boundaries and keeps pushing on something you’ve made clear is non-negotiable.