Like the back of her hand

When a girl realises that she has always loved her best friend, she realises as well that she wants to spend the rest of her life by her side.
or
Isadora has been friends with Megara for two years and she adores the girl.
Everything feels so easy and at peace around her and Isa has no ideas of what she'd do if Meg ever chooses to walk away someday. But for now, Meg is here and if Isa is sure of one thing is that she wants her friend in her life until the very end.