Then she does see you as separate and different.
You cannot be the same and always wrong, if she is always right.
I just want to clarify this apparent contradiction.
Narcissists struggle deeply with other people thinking differently to them.
As children, they were either shamed for their thoughts and desires and made to feel stupid and weak for who they were and what they wanted, or they were spoilt terribly – over-praised and given no or very few limits. Their caregivers tend to have been unpredictable, so they never knew what was 'right' or 'wrong', or their caregivers were deeply neglectful and just let them do whatever they wanted.
As a result, when they grow up, they build their ego and identity up around the idea that whatever they think, like or want is 'right' and 'good' and perfect.
This iseither as a compensatory reaction against their unfair childhood shaming and unpredictability, or as a growth of the idea that they have always been right and can/should have whatever they want. So there's a dual path to narcissism.
This leads to a black and white view of the world. When someone agrees with them, that person is 'like them' and therefore good. When someone disagrees with them, that person is 'not like them' and therefore bad. Narcissists are very rigid in their thinking, because rigidity provides a sense of safety. It's a way of looking at and being in the world that allows them to feel in control. They know what the rules are because they make the rules.
You might have heard that narcissists see their children as 'extensions of themselves'. What this means is that they expect their children to be just like they are – to think the same way, to like the same things, to want the same things. Like small versions of themselves. This is what is meant when we say 'Narcissists can't see others as separate and different'.
It might be more accurate to say 'Narcissists can't bear to see others they care about as separate and different'. This is because, as mentioned above, they have a deep ego need to see themselves as perfect and always right. So if someone they care about doesn't think the exact same way or want the exact same things, there must be something wrong with THAT person. Because the idea that something might be wrong with them triggers deep shame or anger and a sense of self-denial.
Sometimes this 'extension' works in an inverse way. Like, if a narcissist really admires people who are very musical, they might be attracted to you because you are a wonderful musician. They feel that your talent somehow 'rubs off' on them by association. They'll brag about your musical talents, hope (and hint) that you might write love songs for them, engage in the world of music as much as possible. However if you lose interest in music, or perhaps have an injury that prevents you playing any more, this will upset and even anger them. As they have come to feel possessive about your musical talent, they now feel you are denying an 'aspect of themself'.
So that's why when you do things differently to your mother, she reacts very badly. She wouldn't do things that way, so you, as an extension of herself, mustn't. If you do something differently, it might mean she was wrong about what she thought. And if she's wrong about this one thing, she might be wrong about other things. The world suddenly seems unpredictable and scary again. All of a sudden, the entire edifice of her ego is at risk of cracking and falling apart. So that's why she gets mad and flounces off.
It's also why it's effective when you can disagree with her in a loving way that maintains a connection with her. In her world, agreement is acceptance and disagreement is rejection. She tries to force you to agree with her because she feels like if you disagree with her, you'll reject her.