I understand the predicament you're in, however guidance is very clear in these circumstances, you can, and should, break confidentiality and report this from the pov of being a care worker with a duty of care. It's abuse and you have a duty to your client to prevent any further harm to the best of your ability, and failure to do so can have concequences for you - I'm not saying this to scare you or make you feel bad, unfortunately it's the reality of a situation like this.
I understand that if you do, she'll likely shut down and not confide in you again, and quite possibly deny what she's said to you, it's one of the awful situations care workers find themselves in where it seems like a no win situation and that if you do report, not only may the abuse continue, but the abused person will lose the person they've eventually confided in too, so may well continue 'under the radar'. It's also a worry that the abuse may escalate in 'revenge' or you may personally be a target.
In your situation I have spoken to the person who's been abused, and tried to persuade them that the right thing to do is to report this, gently and tactfully, explain that you're there to care for them and this is part of that, and you are duty and morally bound to stop the abuse, and will do it because you care for and about them. It's got to come from the standpoint that it's their wellbeing at the forefront of this, not that you might get into trouble.
I'm really sorry you're facing this, it's a horrible situation to be in.