How can I stop being jealous of others?

I am an eighteen year old professional ballet dancer and I have struggled with jealousy and a lack of self-confidence my whole life. Every time one of my friends has some kind of an achievement, especially in dance, I become depressed with myself and my own life. I also become angry that the other person is more successful than I am and I try to do everything in my power to undermine their achievements. For example, one of my best friends is in a contest to be on the cover of a magazine, and today she came into school and wrote the name of the website where to vote for her on the board in one of our cles. I instantly stood up to write the name of another girl (who goes to my dance studio and is also in the competition) that people should vote for too. My friend became so angry and started yelling at me about how if I was a sincere friend I wouldn't try to solicit votes for others. She also stopped talking to me. Honestly, the only reason why I did that was because I don't want her to win. I dont want people in my school to think that she is somehow better than me. There was also a talent show today and me and that girl (her name is Angela) both did ballet routines. Although I am a better dancer than Angela (I have won competitions while she has not), for some reason she won first place while I won third. I think my stage presence when I danced may have been worse than Angela's due to the stressful day I was facing. When I came home today I cried for half an hour because now people in my school believe that she is a better dancer than me. Although I know that is not true, I wanted to prove my superior dancing skills to my peers and teachers and I am miserable because I wasn't able to do that. I feel like my jealousy is ruining my life. Are there any tips for having a high self-esteem that will allow me to stop focusing on the accomplishments of others? Thank you in advance for any replies.