Why?

Lately, I have been wondering why I have met certain people, and also what decision is the right decision. For example, why do we meet people that are only in your life for a short time, and during that short time they only hurt you? What was the point? Especially if you've already been through so much. What lesson was that one person supposed to teach you? Life can be so hard sometimes because all you want to do is live your life right. God has a mission for me, a purpose and a plan for me, and I am terrified that I am going to disappoint him. I'm trying to keep my eyes open and seek opportunities, but I feel like I am missing them every time. So, again, I ask, "Why?"