Eating one or even two sugar cookies does not make you a bad person or an unhealthy one. That is a perfectly acceptable portion size.
Now, if you find that the cookies are still calling your name, time to take a walk. Out of sight. out of mind. Also, you can always ask your mom to put the cookies away so that you're not tempted to eat the entire tray. I'm sure she's baking those cookies for a special occasion.
So, you've had your portion, time to step away from the cookie tray.