Honestly, I can see the points being made to you here... but in your defense I do understand your hurt.

I know when I pour out our situation to a friend I start to feel antsy about a response. If I can see they've been on FB for example but they haven't replied part of me feels hurt, like "this is a REALLY BIG DEAL to me and it would help if you could at least just acknowledge that with a quick reply". But I always talk myself down because I know I have good friends and they would never intend to hurt me. Their delay in replying is usually due to the gravity of the topic and their desire to give it a thoughtful response so they put it off till they have more time. Due to this I've learnt to only share the bigger stuff when you're going to get an instant response (face-to-face, phone call, even an online chat) or else only when you're not in a vulnerable state and you can live without the instant acknowledgement.

So given your history with this friend and the randomness of your email contact I don't think her etiquette is out of line. I think you need to accept the mode of communication and manage your own level of sharing and expectations for a response.