Do you honestly like your friend and think she's a good person?
Which is more likely:
Someone you thought was nice, that you trusted, that knew about your grief, that was generally seen as a good person, has suddenly become malicious, who would laugh in your face deliberately, thinks your a twat for saying something about grief, thinks it's funny that you lost a family member, and wants to make you feel bad, combined with your amazing mind-reading skills that tell you that you know that is why she laughed, despite never having suspected she was that kind of person before.
Or, someone who is a genuinely good person, but who had a stupid reaction to something - perhaps because it reminded her of something else, or because that's what happens when she feels strong emotion, or because she misheard you, or because she misunderstood what you meant, or because it seemed untrue to her, or she simply made a mistake in judging your mood? And that you, not being a mind-reader, but caught up in your own upset, accidentally assumed that it meant that she thought the situation was honestly funny, and that she must think you are a twat for saying it? And that she was uncomfortable with the whole situation, she didn't know what to do when you reacted, or she didn't realise what your reaction meant, or, she was embarrassed enough that she didn't know how to get out of the situation graciously, and so didn't refer to it again, perhaps?
There are so many other reasons for her laughing that don't mean that she thinks you are a twat for saying something, or that she thinks the situation is funny. If you are prepared to lose a friendship over it, without even telling her that you found her laughter hurtful, let alone trying to understand and forgive that she might not have meant it the way you assume she did, it sounds like you really don't think very much of her.
I'm sorry you are having a hard time over the anniversary. Perhaps you can explain that to her, and reassure her that you realise she didn't mean it to be hurtful, but explain how you would like to be supported instead, and what she could do to help? If she is a good friend, then she will probably try to help in the way you would most like.