You can’t go back in time to deepen your previous friendships but you can grow from this realization and move forward. Your next big birthday is 21 (at least in my country) but you could choose any age/timeframe you want. Just make yourself a goal: by the time I turn X years old, I will have 2-3 really good friends that I love and love me back.

And then work toward that. Discard people who aren’t interested in you and find people who are, and build those relationships. Work on them.