Came out as bi and that wasn't the whole truth

I told my mom about a year ago that I was bi. She was surprised, but not upset. She didn't get it how I could like both boys and girls and I just shrugged and said "It happens."Well, that wasn't the truth. I exclusively like girls. I cannot ever see myself with a man. I want to tell my mom, to tell her I am actually a lesbian but I don't want her to be mad. I feel like she might this time around just because she hates it when I lie to her.How should I go about this?