If you can't change a situation then all you have to do is change your attitude. The biggest waste of time in our life is worrying about things over which we have no control.
Focus on the things which you can control because that's the easiest way to change your life.
The only way to stop overthinking is through action. period. Inaction is a breeding ground for fear, anxiety and stress. But at the Same time action is a breeding ground for hope, optimism and happiness.
Always keep in mind that both good and bad times are inevitable. So don't worry too much about inevitable things. All you have to do is just take one day at a time and focus on just one goal and put your 100% focus and attention towards that goal.
When we remove all the clutter from our lives and make it very simple that's when we rise above the fear. Never in your life destroy your present for the fear of the unknown. When you are walking through hell then all you have to do is keep walking.