Changing negative thoughts into positive thoughts is like the sun rising in the morning. When the sun rises it kills the darkness. The positive kills the negative. There can not be darkness where there is light. That's why negative thoughts live in the shadows. They are always there. As long as you don't shine any light on them, they'll stay there.
It's hard not to entertain those shadowy negative thoughts. I understand that. Look at it like this. If you walk into a dark room and can't see a damn thing, your first thought is where is the light? Now, picture entering that same room, but this time, the light is on. You don't go, hey, where is the dark? No, you don't! You just keep moving. If you could keep the positive thoughts shining then the negative thoughts will not appear.
What if you were able to look at life like that? Never looking for the dark, always looking for the light. If you can go every day asking yourself where's the light? Then life will be bright all the time. Where is your light?
You've heard that thinking negatively is easy. I get that. It's hard being positive when everything seems to be going against you. It's hard to get up and find the light when you want to see where you are going! Or is it? Remember the dark room is only dark until you find the light and turn it on. Entertain the light. Let the sun shine! Open your eyes! Where is your light?