How do I make grant and competition pitches that actually win?

You are already applying, which many people never work up the nerve to do. The gap now is between entering and winning, and that gap is normal. Winning is a skill of its own, separate from having a good idea, and almost no one is good at it right away. A winning application does one thing above all: it makes the judges quickly see why your idea deserves their money over the others. That means being clear about the problem, concrete about your plan, and honest about why you can pull it off. Study the ones that won before you. Notice what they emphasized and how plainly they said it. Most losing pitches are not bad ideas; they are good ideas explained in a fog. Before your next application, find one winner in your category and study what made it clear and convincing. You are not far off. You are learning to say a good idea in a way that lands, and that skill grows every time you try.