There are many benefits to positive thinking, but negative thinking has it benefits too. For example, when you design a product or launch a company, you need to believe it will succeed, but you also need to think of everything that could possibly go wrong so you can prepare for and prevent it. That's a good balance of positive and negative thinking.
Unfortunately, for many of us, positive and negative thinking can fall out of balance. Some people might be overly optimistic when some negative thinking would be useful. Fortunately, optimistic people bounce back from problems easily because they know the next time will be better. On the other hand, pessimistic people who encounter problems find it harder to bounce back because they expect the next time will be the same or worse. This can lead to further negativity.
Negative thinking can lead to excessive self-doubt and self-criticism. Self-doubt can be useful. If you think your public speaking skills aren't up to par, you might be motivated to work on improving them. Self-criticism can help you refine your techniques. But excessive self-doubt and self-criticism can lead to an erosion of self-esteem and self-confidence. Negative thinking that continually undermines your ability to value and believe in yourself is not productive and can lead to depression. You can halt and repair the damage by replacing negative statements with positive statements.
This may sound too easy to you. It may sound like it would never work. The fact is you may have already spent many years telling yourself negative messages or hearing them from someone else. Repetition bypasses our critical thinking and gets ideas into the subconscious. This is why commercials and nursery rhymes are repeated over and over. The good news is that repeating positive messages to yourself can override and rebuild the confidence that has been eroded. Hypnosis and hypnotherapy can speed up the process considerably.