You really aren't stupid. It was a reasonable mistake to make. They are packaged together after all!
The reason that serving sizes seem so small is because the present standards were determined about 40 years ago from surveys when people consumed less food. Even though the serving size is regulated we all know from observation that the food companies take advantage of the outdated regulations.
Poptarts are really awful for you, but you know that. Even switching to the "fiber" poptarts would be a lot better since they have more fiber and protein than a normal one. Maybe crisp diet / whole wheat toast with a tad of butter spray and no-sugar, low sugar jam/jelly or cinnamon and sweetner you would eventually find satisfying?