How can I control myself!?

Now, I'm in grade 12. I thought I could control my self and my feeling because I've been there for a year.However, The answer is opposite my thought. I get a lot of pressure from my friend. I have one girl-from Venezuela . I thought I and she will be good friends but, I am wrong. When I talked to someone, she tried to against me, every time. I'm so tired now. Also,grade 12 get me more pressure. How can I control myself!?