Three thoughts: 1) the more you practice descending, the more comfortable you'll become with it, 2) the further down the road you look, the better off you'll be, and 3) practice descending on a better road!

Honestly, if I had a choice between dodging cars at 60mph and and riding on a poorly-maintained shoulder I'm not sure I'd ever be comfortable. Your best bet might be to find someplace better to practice...