There is more variation in body shapes of women than for men. So some styles work well on a certain body type, not so well on others. So of course it takes more trial and error in finding the right look. To me that makes it more fun!
BTW, I am the same height as you and I weigh around 300 lb. I share your frustration and pain. If I find two dresses in a season that I'm happy with, it's been a great season. That represents a lot of shopping trips, but it makes the victories sweeter. And I've made friends with a lot of sales assistants.