The sweetness of your sauce depends on your ingredients. Assuming that you aren't adding any sugar, the sweetness is probably coming from whatever tomato product you are using. Check the label to make sure that they don't add any sugar to it, and try different brands. Generally tomato puree will be less sweet than tomato sauce. When I was using canned tomatoes, I usually bought the crushed tomatoes, but the brands vary so you have to read the labels. I like to use Pomi boxed tomatoes, because nothing is added; but make sure you don't get the marinara by mistake.