Most people who are negative believe that they cannot get a positive mindset. They think it's too hard, that it's not them, or that if they 'pretend' to be positive they are just being fake.
You see, if you aren't positive, it is ingrain in you that you are a negative person. It Just who I am, you say to yourself.
There's a difference between believing that sometimes you have negative thoughts and actually believing you are a negative person. If you believe that you are a negative person then it'll be hard for you to change.