You do what 99% of the people in the world have done when the person they like doesn’t like them back. And that is, get over it and move on. Sure, it is disappointing. But all the disappointments we face are simply part of the experience of life. And with each experience, we all become a little more resilient.
We can’t go through life expecting everyone to like us and everything to go our way. (It is kind of cool to learn that when we’re young. Toughens us up and prepares us for the big, wide world after graduation from school.)
When you walk down the street, every adult you pass will have liked someone (probably more than one) who didn’t like them back. Even people who are madly in love with a wonderful partner today, had their hearts broken at some time in the past.
The popular boy in your school is not going to be the love of your life. The love of your life (when you eventually meet him) will be SO much better than the kid who doesn’t like you.