Why did cassandra from the Trojan war consider her gift to be a curse?

No one believes whatever Cassandra predicted. Basically, she sees these horrible images of what was going to happen to Troy and people she know, but everyone just thinks she is crazy. Some of Cassandra's predictions were that Paris will bring the destruction of Troy, Priam bringing Hector's body back from Achilles, the Trojan Horse, and Agamemnon's violent death.