Idk if you can stop thinking negative. But you can start thinking positively. Researchs show its easier to learn a new ability than to stop bad habits. Whenever you see sth bad, dont hinder your brain when it thinks negative stuffs but after that start thinking abt positive things. The beauty of this is that negative thoughts have no power when it is related to the positive one. Roughly positive thoughts are 100x better. So your brain will start to think positively. Step by step