Sometimes your story doesn’t want to heed to your plan, sometimes you get half way into your story and you feel it going somewhere else. Let it! Let your story guide you where it needs to go. Because if you force it back into the plot you’ve planned, you risk making it harder for you to write and that takes away the richness of your story.
Sometimes your characters end up doing a full 180 degrees on their characteristics. That’s okay! Your villains can find redemption. Your heroes can fall from grace. Your characters can lose their optimism, they can become corrupted and immoral, or they find hope. You characters can change, they can develop and grow!
Sometimes things change, sometimes your story evolves as you write. That’s okay. Don’t let it stress you out.