I know I am in the wrong

I didn’t tell my girlfriend what time I was getting off and I assumed I told her by saying “I remember telling you”. She said that’s very manipulative(which I don’t disagree with). How should I go about apologizing because she said I don’t want you when I apologize and I am trying to be and it’s hard because my voice it’s very monotone most of the time so I don’t know how to sound apologetic. I’m particularly in text or even when I call her she says that.