The premise is wrong. It starts with negativity but then switches to sadness. Negativity is a perception and a reaction to a situation. Sadness is a feeling. A person can be sad and have a positive outlook.
You can absolutely just change your perception from negative to positive. It takes work and you have to be aware but it can be done. Sadness however can’t be willed away. But your response to being sad is under your control. 