You my friend, are suffering from a classic case of target panic. I've been battling with it for about 3 years now and it's definitely not an easy thing to get rid of. There's been a couple good threads over the summer on that topic that you might want to reference back on.

This one comes to mind: Target Panic Thread

While I never quite took the time to go through some of the more elaborate steps that some people suggested, I found the thing that helped me the most was simply creating a sequence of events in my mind for every time I shot. I would recite these events in order every time I shot, over and over and over. For me it was 1. Draw 2. Anchor 3. Aim 4. Hold Steady 5. Squeeze. I took my sweet time every time I shot to really emphasize the "Hold Steady" and "Squeeze" parts. If I couldn't hold steady or I felt like I was going to flinch and punch the trigger, I would simply let down, take a deep breath, and try it again. After awhile it all starts becoming a habit that is engrained in the way you shoot and you stop making those flinching mistakes.

Hope that helps.