Majority of PTSD patient loss some part of their memory especially from the traumatic incidents. Their brain will bind them to remember the terrifying thing had happened. it's the same as how serotonin works on your body. it's probably why his dad never told him about his mother. from what it looks, Qingyu is definitely had PTSD from her mother's death incident.
what ptsd? a deasese from the mother?
from google : Post-traumatic stress disorder (PTSD) is a mental health condition that's triggered by a terrifying event — either experiencing it or witnessing it. Symptoms may include flashbacks, nightmares and severe anxiety, as well as uncontrollable thoughts about the event.
why can't yu remember anything? is he had amnesia?