Some people miss the point of the story. When they call some one a “Cassandra” they mean that the person is just a pessimist, always predicting the worst. That is probably the use to which you refer.
To be called a Cassandra is a compliment. It implies that you are an accurate forecaster, but people round you will not listen. Apollo gave her the gift of prophecy, but she rejected him. So, he cursed her by making sure no one would believe her true prophecies.
Cassandra had a miserable life. No one in Troy believed her, and she was treated as mad. As she predicted, her city was destroyed and her family killed. She was carried off by King Agamemnon, as a souvenir of his trip to Turkey (Asia Minor).
Agamemnon’s wife Clytaemnestra was not best pleased with him. He had sacrificed their daughter Iphegenia to get a good wind when he was sailing to Troy. Then, he stayed away for ten years, before returning home with a new concubine - Cassandra.
That would put a strain on anyone’s marriage. Clytaemnestra killed both her husband and Cassandra. As usual, no one had listened to Cassandra when she predicted it would happen.