How can I finally be happy?

I'm 16, stressed about school, self conscious, and unhappy. People always tell me I'm pretty but I truly don't believe them. I plan on being an animal rescuer when I grow up, but I am soooo stressed about school. I'm supposed to be writing an english paper as I'm typing this right now. I go to therapy every monday trying to deal with stress and pressure, but I don't know how well it's going because the sadness and stress never goes away. I just want to be happy with my life. I have amazing friends and a great family, but I can't stand all of the work I have to do and how bad I always feel about myself. Any suggestions?