Lots of people struggle with the backwards gliding in Level 3. You are not alone.
You've identified a major part of the problem, which is almost certainly your weight being in the wrong place. Try to bend your knees more, ensuring that you keep your weight (ie, your hips, shoulders and head) all in a line over the middle of your blades. If it helps, try doing backwards swizzles/lemons, bringing your feet parallel between each one and then lifting alternate feet for a one-foot glide on the flat of the blade between each swizzle. As you get more confident with this, feeling your weight being lined up over the middle of the blade, you should be able to adjust the exercise so you push more with one foot and glide on the other, eventually moving the weight enough that you glide on a backwards outside edge but start off with the swizzles. Rome wasn't built in a day. Skating backwards is more about transfer of weight than anything else but until you get confident enough to do it, it's a tricky one.