I thought I was poly but I really don't like when my partner is seeing someone else. She broke up with him and told me it's my fault and now I'm feeling so guilty. I don't know what to do.
You don't control your partner's actions. You told your partner "I don't like when you're seeing someone else." She could have decided to: A.) Stop seeing you so she can keep seeing other people, B.) Stop seeing that person so she can stay with you, or C.) Keep seeing both of you and try to make it work.
It sounds like she chose B, but is shooting herself in the foot now - why sacrifice one relationship for another, only to sabotage that one with guilt and resentment?
It's okay to tell your partner that you don't appreciate being made to feel guilty or at fault for feelings that you can't help and choices that you didn't make.
It may be that you two are not well suited to date each other, because you don't want to date someone who dates other people, and your partner will feel controlled and resentful if she dates someone who doesn't want her to date other people.
It's also okay for you to decide that you don't want to stay in a relationship characterized by guilt and blame. If your partner is going to respond to you sharing your feelings by behaving like this, she is making it unsafe for you to be open and honest, and that's not fair or healthy.