I'm going to flake on my best friend
Why would you want to flake on your best friend?
Because I want to give her time to notice what I see in her.
I can see your point, but it is wrong to break a date with your close friend. They will feel hurt and won't like to hang out with you anymore. You should spend time with them and make them feel appreciated.
I want to do this she needs to think about what she has done.
If you don't tell her why she might never be able to change. Keeping your word even if you friend is wrong can show how much you actually care for your relationships.