Jealousy problem!! Please help!!!!!!!

I know it's wrong to be jealous of someone and I'm ashamed of it but I was wonder if anyone knew any ways to make sure it doesn't get to me. The problem is that the boy I like is always flirting and hitting on my best friend. I always feel like I'm not good enough or that something I do is a turn off. Any suggestions as to how I won't think of these things? Or to help me not be jealous?