You've gotten great advice so I just wanted to send my support Introducing cats is incredibly stressful on us too, don't forget to take care of yourself and give yourself breaks! You're doing an amazing job with what seems like a difficult match, and most importantly you've opened your home to two cats in need. Sometimes these things take an exhaustingly long time, but with Mia being so young, I think it's more likely than not that there's going to be a time in the future you look back at this thread and can't believe the two couldn't co-exist. Though it probably seems like no progress is being made, just remember that one less hiss or swat is progress to your cats!