I didn't notice a drop in supply at all. I weaned DS at about 9 weeks pg coz I was just too sensitive.
I also suffered HG. More in my first 2 pregnancies. This one as well, but not quite as bad.
Just try keeping your fluids up. Anyway you have too. Cordial, juice, anything. Eat what you can. It is hard, but your body will take care of the pregnancy first. It'll be harder on you than anyone.
Goodluck x