Jeremy said in his interview transcripts the relationship had been in decline for around 6 months. The judge confirmed this in his summing up when asking why Jeremy would bring a bike to his cottage if the relationship was coming to a close.
Julie did not like Brett much & did not want to a to go to some places with them. But Jeremy persuaded her. Jeremy also persuaded her to attend the funeral with him.
Jeremy gave himself no brownie points saying to Julie he hired a hit man to massacre his family. Asking another woman out in front of Julie would have done him no favours either. Julie hit him. She also said herself she put a cushion over his head. Then took it off.
Wrestling with what she knew from the 3am phone call & Jeremys confessions afterwards, Julie would have been pleased to escape.