If you carefully observe the people who are in stress, you will notice that they keep thinking about a specific event, conversation, failure, brawl or hard luck again and again which has happened to them in past. Such people do not have control on their thinking process hence they allow their brain to keep wandering in the past. While It's a known fact that bad luck happens to everyone, it's people's attitude and the quantum of reaction which defines the degree of stress. Since stress and happiness are vice versa so more stress results in less happiness or in some cases loss of happiness.
Always remember that everyone passes through tragic moments in life however you will agree that the more you think about bad things happened in the past, the more magnitude of stress you give birth to. Hence the chains of negativity and stress around your neck need to be broken and thrown away so as to move on with your life.
The fact remains the same that you cannot change the past, neither you can boil the ocean so a wise person would not keep thinking about something which has already happened instead he would learn the lesson from the past and take measures which need to be done in order to prevent such happening in future. A wise man will always focus on the solutions instead of thinking about the problems.
Take a pen and list down all the issues bothering you right now. You will, eventually, end up with a very limited number of issues causing depression back and forth. Now the next step is to split these issues into two groups. One group for which you do have some solution and the other group should contain the issues which are out of control. The worries and sorrows from the second group should be shun away. You need to train your brain not to think of something which is beyond control. Divert your focus on the issues for which you can do something. Do it now because this time will also become past and you would be cursing these moments for taking action at right time.