Stressing over something is completely normal and a part of life. It is something that happens like everybody and seriously there is no one who can say that they have never been stressed out. What the real thing is that do not stress about things so much that it starts to interfere in your life.
What I mean is the fact that stressful situations are hard to cope with up but it is not impossible to control. Why because the amount of stress you take or face is all up to you. You are the controller over your mind and if you let stress get control over your mind then it becomes a huge problem. Why are you stressing all the time is because you are unable to let go of the tension or worry that is hanging over your head like a cloud.
For you, it is near impossible to block out worries. But here is the deal you need to learn to block over those things because life is not easy and facing something and learning to cope up with it is the one thing that keeps your sanity tackled because if you cannot then I am damn sure it will haunt you in every step of the way.
The key is to accept. What has happened has happened or what will happen will happen. Fretting about such stuff would not alter the course of your fate. You need to learn to accept and move on and let things be… You need to find a way around the problem but not go inside it. That is how stress will be at bay.