im obviously not a pro at this. ive never been in an abusive relationship, so it would be difficult to give advice regarding one. however: ive noticed a LOT of people who like to write abusive relationships, and its getting to be a bit much. please dont romanticize abuse.
if you are going to write an abusive relationship, try to make it realistic and maybe do some research
research things like PTSD, general trauma and how authorities would handle the situation.
do NOT write something where a character gets in a relationship and they are suddenly "fixed". thats not how it works! obviously other people might HELP overcome someones trauma but like come on. no one person can "cure" someone else of a mental illness like ptsd or of their past trauma in general. if they think they can it is a very toxic mindset to have- so then consider how other characters interact with them! what is their mindset? do they consider themself a "fixer"?