It's not your imagination. It takes a long time for kids to develop adult levels of self-control. Bunge and Wright have an excellent review in Current Opinion in Neurobiology entitled Neurodevelopmental changes in working memory and cognitive control. Some of the key figures can be seen below. The limitations in working memory for children remind us that we need to take care to break down new information into smaller "bits" and provide plenty of supports to working memory (e.g. handouts, listing of key steps, examples of correctly solved problems).
Self-control of emotion is a different issue, but it also has a developmental course and is affected by intrinsic (innate) and extrinsic (external) factors. For a nice review, check out the second reference at the bottom.
Neurodevelopmental Changes in Working Memory and Cognitive Control pdf
The Development of Self-Control of Emotion pdf
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