When you are using your rotarty cutter and ruler...
Line up the ruler and place your finger tips on the ruler and press downward.
Hold the rotary cutter against the ruler and straight up and down, not angled, you dont neet to see where it is going, it follows the ruler.
and
Push down on the rotary cutter, not against the ruller. It doesnt take presser against the ruler to go straight, the ruler is a guide.
When you cut past where your hand is holding the ruler, stop and walk your hand up the ruler and push down again, so you always have control of the ruler where the cutter is cutting..
Practice will make you better
Hope this helps...