trouble with finding friends

I am in high school and I was dating a guy for over a year. It was a bad relationship and he was abusive. While we were dating people were out making friends and connections and hooking up with other people. After a year and we are no longer together , everyone already has set relationships and friends and I have no one. How can I get back out there and make friends when friendships and groups are already made ?
Also now that I am single people are now getting into relationships and no one wants to get with me. How can I branch out ?