it must be hard for you but i would ask her not to bring him home,out of respect for you.and to move out if possible.it's hard for alot of people when their ex's start dating again especially after a month.
i rang one of my ex's once after about 2 weeks of leaving her to arrange picking up the rest of my stuff and she gloated on telling me that she was in bed that moment with one of her ex g/f's lol so just for a laugh i shouted loudly down the phone "but u said u still loved me"pmsl.i couldn't had cared less even if she had the dagenham girls piper band in bed with her,she was a super bitch.
my advice would be she has shown you no respect or feelings so just try and forget her i know it's hard ok cos she's living there.go out and enjoy yourself.but also think if you meet the girl of your dreams how will she accept if your ex is still living there.take care.

hi.when my ex and i finished,we tryed living together for the sake of our daughter.he brought his girlfriend back and she stayed over night.i did the same with my girlfriend.it didn't really work that well because my ex and i both felt guilty.he moved out,now i live with my girlfriend and 2 kids.i still get on really well with my ex,he's a good bloke.