How do you stop stressing over having a baby?

My husband and I have been "trying" to have a baby for a year and a half now. A lot of women I talk to tell me it took them 3 years until they conceived. The only problem is I think about it everyday and I just found out that another one of my friends are pregnant. All of my friends have been able to have kids but me. I know we haven't been trying for that long, but it's really heartbreaking to hear this. I am very happy for my friends, but jealous and angry at the same time. I really wish I could find the off switch in my head so I quit stressing over it. Any suggestions??