This, my friend, is the human condition. When we're children, a summer can seem like forever. At my age (80) the years fly by.
Think of it this way: When you're ten years old, and somebody tells you that you can't have a certain thing you want for another year, well, that's 10% of your life. But at the age of 80, that would be only a bit over 1% -- not nearly as long.
So, how to make life seem longer? Pay more attention in the moment. Don't take routine experiences for granted. Learn to see everyday miracles as miracles, not just the same old same old. Some people would call it mindfulness, but since that's become a controversial term, don't mess with it, and for heaven's sake don't take a workshop or follow some self-help book. Just experiment with how you, in the life you currently live, can be more open and aware of the world around you. It's fascinating, and time will slow down.