It's so easy to think negative. So easy. I think we have created a habit of doing such that we don't even notice when we're being negative.
When a negative thought comes understand what is affecting you to have such thought. Feel unworthy? Something from your past? You need to be aware of the negative thought because it produces more negative thoughts. Once you recognize why, change your thought into something positive. It's like turning "I can't" to "I can".
Having more positive thoughts creates positive manifestation into your life. You'll be happier and you won't worry about what you can't control. When you have negative thoughts it brings negative emotions like fear, sadness, and you tend to over analyzed situations. When producing positive thoughts, you'll be present and will understand that everything is working for your good. When you see others thinking negative try to reassure them with positivity. Don't entertain it.
Thinking positive can be a daily struggle for me. As I continue to affirm and pray over my life I begin to develop more positive thoughts. I don't worry as much because I know everything is happening for a reason.
To me being positive is equivalent to having faith. The negative thought is the doubt and the positive thought is the faith. Do you believe in your process of life? Believe that many things are temporary?
Don't dwell on the negativity, it's not good for you or others around you.