Following up kinda-sorta on the same path Bubbles went on....
Are you absolutely positive you have control of your horses body?
Selena will often try and cheat me when we first start on the pattern by either blowing out or shouldering when she feels a little disobedient. Which is why I always start my rides off with counter-arc circles, haunch turns, and rollbacks to make sure I have control of the shoulder. This always tends to bring the horse's mind back to you, loosens them up, gets them thinking a little. You have control of the shoulder, you have control of the horse.
Be sure when you go into your turn, you're about three or four feet away to start the pocket, then pretty close coming out. Use inside leg about three strides before the turn to establish some bend, then sit and support her with your inside leg but use your outside leg to really bring her shoulder across the finish the turn. When you are schooling, if she tries to blow out, I would break the gait, haunch turn her into the barrel, then start circling it the opposite direction. When she goes out fine, let her go to the next barrel, stop, and rest.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.