Can you quickly change your way of thinking or is it a process that takes time?

Usually changing the way you think is a timely process.  First, you need to realize how negatively you think before you can initiate the process of starting to think positively.  Next, you need to realize when you are using your negative thinking the most.  Is it during a time of high stress? Is it in a time when you are faced with a challenge? The next stage is replacing the bad with the good.  You have to actively realize when you are utilizing negative thinking and replace it with positive thoughts.  An example might be "There is absolutely no way I can do that," when you should be thinking "I know this may be challenging, but I'm going to give it my best shot."

To recap the steps to changing your thinking include:
1. Recognize
2. Be aware
3. Slowly change and introduce a new thinking pattern
4. Keep working on all your destructive thinking patterns

Source: ww.stevenaitchison.co.uk/blog/8-destructive-thinking-patterns-and-how-to-change-them/