I finally found the 7-step poem again, in ep4. YZ teaches it to CY in despair after trying to get her to memorize…
Ohh it's the poem written by Cao Zhi for his brother emperor Cao Pi who was going to kill him after the 7 steps if he didn't get a good poem within that time as an excuse. The poem made him emotional and he decided not to kill his brother. Although it didn't quite match Changyu's situationπ