PTSD... will it go away on its own or only with help?

My wife and I went through a frightening traumatic event together. Its been four months and my wife seems to be fine and really never seemed to have too many lingering effects from that night, I thank God for that! But I still suffer having sad, depression, hopeless feelings mainly of the future. I look at the future with little hope and know that someday our wonderful life together will end and I will p or my wife will p and leave the other one alone heartbroken. It brings me down to tears at times. I suffered with this for 2 months after the incident and then it seemed to go away for the most part for a few weeks. Now for the past couple weeks it seems to be taken hold of me again. Is it normal to go through this and have it return for a bit? Can I expect it to go away with time or am I doomed with this forever? Should I give it a little more time or is it time to get help?