The speaker admits to having broken "the other" inside themselves, some second self that didn't fit. The title, "Autotoxicity," suggests this was a kind of self-poisoning: the harm came from within, not from outside. What got broken seems to have been sacrificed to the "expected story of a boy," the script of how a boy is supposed to grow up.
The horse images are strange and compressed. "A near horse / a fear horse" plays on sound, one word sliding into the other, so closeness and fear feel almost the same. A horse can stand for something wild or hard to control, and here it seems to be the part of the self that was held at a distance, or tamed out of fear. That reading is an inference, since the poem never explains it.
The ending turns gentler. Saying all this aloud brings no pain: "there was no hurt / in talking." The mood is quiet and wary, but it ends with relief, as if naming the damage is a first step toward easing it.