For overthinking: keeping yourself busy with other things. Its hard think about the other things when you get sucked into a video game like Fallout 4 for example. Focus on your hobbies and health and stuff.
Meditation can have its benefits for quieting your mind.
Recognize and identify these intrusive thoughts before the cycle begins.
Yesterday already happened and tomorrow isn't here yet. Focus on the present moment. When you're at work, be at work. When you're at home, be at home.
Worrying about something is contemplating an issue without thinking of a solution. So why waste two seconds of your life worrying about something you can't fix?
"Change only takes place through action… Frankly speaking, not through prayer or meditation, but through action."
- Dalai Lama
Fear is: False Expectations Appearing Real
My favorite quote is, "just fucking do it"
Don't get paralyzed by "paralysis by analysis"
"Sometimes fear can be a good thing - keeps you from taking unnecessary chances. Courage doesn't mean that you don't have fear. It means that you've learned to overcome it."
- Lieutenant Tom Paris (Star Trek Voyager)
Don't listen to the whisper, thats the whisper of 10,000 generations of human tribalism. When we lived in a community of 100-150 people, it was bad to do different or embarrassing, but today that strategy is antiquated.
Fear is just chemicals in your brain. It's an animal instinct. Ignore it and you will find strength.