She's not haunting you. She can't hurt you anymore.
It's simply your mind's way of dealing with the grief and the unresolved issues she left you with, and the flashbacks of trauma.
The dreams are your way of processing all of that.
It's your way of trying to get some kind of closure.
Let yourself have the dreams (or nightmares) without any judgement and without trying to stop them, they will resolve on their own in time.
Don't be surprised if you have a dream one night where you suddenly resolve your relationship with your mother. It might start out as a nightmare, and end on an unexpected pleasant note. This is your brain's way of getting some sort of closure that she should have given you when she was alive.
Sorry you're going through this. Be strong and take each day at a time.