Lots of alcohol + frustration at not being able to get to her ex's apartment or find him. I don't think the car itself had anything to do with it.
Personally, I really don't believe that the ex had anything to do with it. By all accounts of the timeline and her actions, she was the one who was pursuing him, or seeking him out, for whatever reasons. Maybe she is the one who didn't want to let things go. He sounds like he was in some respects just trying to get away from her. If he was a more typical domestic violence killer, he would have lured her back to his place or somewhere secluded, and killed her there and then got rid of her after the fact. He wouldn't answer his phone, wasn't responding, to me it seems he just didn't want to deal with her. Maybe she found out that he had a date that night and it sent her into a rage. All her actions that day seem impulsive, random and unpredictable, and certainly driven by alcohol consumption.
If foul play happened to her, I don't for a second believe that it was planned. Opportunistic taking advantage, which would be really hard to solve. Where do you even start? I am more in the death by misadventure camp - she fell somewhere or passed out somewhere where she just cannot be found. No matter what, this case is tragically sad, and I am not hopeful that under the circumstances there will ever be answers. I hope I am wrong.