How can I work on my social anxiety?

Until about three years ago, I was very social, going out with my friends, having good conversations and comfortable. But, recently, I’ve had a problem with looking people in the eyes and holding a conversation with someone—even my close friends.
Last night I went out with my close friend to a concert and we talked to a few people but I had a really hard time keeping eye contact with anyone, I could feel weird looks my way and I just want it to stop. Any advice?