Poly has two sides and one is easy, which is dating multiple people (not at the same time though), and the other can be hard because it might trigger strong feelings of abandonment or replacement. So it isn't exactly like trying alcohol or sex for the first time although dating multiple people ethically for the first time can be, but not when they start dating their own partners.

I think you might have extreme abandonment fears which is why poly feels so repulsive to you and it also explains why your abusive ex used poly imagery to traumatize you so deeply. I think you would benefit from therapy after this relationship because you have been with someone who intentionally wanted to hurt you, which is frightening in itself, and because you have effectively been abandoned in your relationship by him. Abuse from a trusted partner is a form of abandonment. You shouldn't seek poly unless you really want it for yourself. You will find mono people who will love you for who you are. The whole world is mono. You will find someone. But focus on working on your trauma first.