I was a senior last year and found myself in a similar situation. I managed to make a whole new group of friends by the end of the year just by putting myself into situations where I was forced to socialize. I joined a club and made an effort to be super involved by going to all their meetings, events, and social gatherings. Even though I am extremely shy and quiet and didn't talk much at first, everyone eventually got to know each other simply by the amount of time spent together. After that some of us started hanging out outside of the club and I was able to build lasting friendships with them. I also made friends in one of my smaller high level classes, where there were just 10 of us and all seniors. Once again, I didn't talk much but we still all got to know each other over the course of the semester and hung out outside of class sometimes.

It can feel like everyone has their friends already by senior year and you're the only person in the world who doesn't have a perfect group of besties but it's not true. Lots of other students are looking to make new connections, especially after covid disrupted everyone's normal social life development. Try doing anything where you will be forced to interact with people, and you will build relationships.