I would offer a different perspective. Don't try to replace the negative with the positive, because then you're just starting an inner war and hoping the right side wins. Instead, practice listening to the negative thoughts without arguing with them or agreeing with them, without resisting them. "It's OK for you to say that because it's your own opinion" is a good start - it means that you're disidentifying from the negative voice and remembering that you are not it. Once you can do that consistently, the voice ceases to have any power over you - it's just words, it's just a stream of data and energy, like birdsong outside the window.
When you can consistently accept the negative voice without judgement, it will weaken, because it needs you to feed it by feeling bad about it. The more it weakens, the less power it has to influence you. And the more it weakens, the more space is left for all those positive thoughts to arise out of the depths of your being unopposed.