Identifying the Nuance of Nostalgia

In the following passage, a character reflects on their childhood memories of summer:

Every summer, my family would escape the sweltering heat of the city and retreat to our cottage by the lake. I remember the air thick with the scent of pine and the exhilarating sense of freedom as we splashed in the cool water. Yet, nestled amidst these joyous recollections lies a lingering sense of nostalgia, a bittersweet awareness that those days were fleeting. As children, we were blissfully unaware of the passage of time, embracing each moment as if it would last forever.