To start on the road to mastering the skill of positive thinking, it is necessary to get rid of all traces of doubt and negative thinking as soon as possible.
If you keep doubting yourself, you will never amount to anything in life. You will probably end up living a mediocre and unhappy life. Too scared to step out of your comfort zone. Too scared to face the reality that failure is a necessary - and important - step towards success. Doubters will doubt themselves. They will doubt the people around them. They will doubt everything and everyone. These people are depressive. They literally make everyone around them suffer just to be around them.
Like most things in life, overcoming doubts and negative thoughts is easier said than done. Sure, it's easy to utter positive words when you need to. But deep down you know your first instinct is to say something gloomy or depressing. You need to put a stop to your destructive inner voice right now.