TL;DR: (content warning for substance abuse and mentions of murder/suicide)

Jet broke up with her because he found out she cheated on him with his best friend BEFORE they were messing around with potentially becoming polyamorous with his best friend. She lost Jet and also Bruce (Jet’s best friend) and her mental state deteriorated (she already had a lot of issues before hand) and started doing hard drugs more often, got involved with the wrong crowd (even more of a wrong crowd than drug dealers) and got her hands on a hell relic dagger. The dagger, although not evil, is both influenced by and will influence who own’s it’s emotions and thoughts and it started to influence and enhance her intrusive thoughts until she ultimately came to the conclusion that the only way to ‘fix things’ was to do a murder suicide of killing Bruce and Jet with the dagger, and then herself.

It gets more messed up then that, especially how she went about trying to do it, and she did succeed in murdering Bruce and nearly got Jet too. It’s why he has that large scar on his abdomen. Also, Jet and Tiffany’s relationship was not healthy in the slightest even before finding out about the cheating. It was very toxic and a lot of Jet’s trauma is because of her and situations she put him in.