Summers felt endless when you were ten because everything was new. You can still get that back by trying new things.

Childhood felt long because almost nothing was routine yet, and every new thing had to be handled with full attention. As you get older, more of the day runs on patterns that require nothing from you, and those hours pass without registering. The speed is not really about age, it is about familiarity.

Rather than trying to recover a feeling, change the input. Learn something clumsy, go somewhere you do not know the way, talk to someone outside your usual set. Attention comes back by itself when a situation genuinely asks for it.