I think you could definitely drop down a few centimeters, especially if you are having a hard time turning. Try looking for a women's all-mountain board in sizes in the low or mid 150's. Not all boards will come in sizes that long, but you should have enough to choose from. Look at boards like the Ride Solace, Rome Vinyl, Burton Lux, or K2 Luna. You should be able to find all of these boards on sale within the next few weeks.
When I learned to ride I had a hard time learning to turn. Getting a snowboard that was softer and shorter made a big difference. But keep in mind that falling while attempting to turn is to be expected for beginners. For me, it was the hardest skill to learn!
Your boots might be a problem as well. The Burton Casas are VERY SOFT beginner boots and may not be stiff enough to support your weight.
Suggestion: Post this in the equipment forum for "boards" and you'll get more help. A lot of people probably don't check this one as frequently.
Good luck and welcome to snowboarding!