The amount isn't always a true indicator of how bad your WD's will be, but if you are drinking 2 40's a night that's at least 6 or 7 units of alcohol, maybe even more if it's a high-alcohol malt liquor/high gravity beer. By most accounts that means that you are binge drinking every day, so your body most likely isn't going to like it too much if you take it all away at once.
You said that you told your doctor that you planned on quitting - did she give you and advice as to how to handle it?