How do I stop feeling jealous in my relationship?

My boyfriend and I have been going out for about a year. I suddenly find myself jealous when I see him talking to other girls. I have never been this way before. I don't like these feelings. He said that my jealousy would cause our relationship to have problems. I told him I can't help these feelings. I don't know where they are coming from, but this is how I feel, I tried to stop, but I can't. I really like this guy, but I'm afraid that I'm messing up the relationship. It's just stupid.