How do I deal with regret over the business I never started?

You had the idea and the opening, and you did not take it — and now you watch someone else living out something like the path you left on the table. That is a particular kind of ache, because it is not envy of luck; it is looking at a version of your own life that you can see clearly and cannot reach. But be careful with the story you are telling. You are comparing your real, complicated life to their highlight, and you are assuming their success would have been yours. It might not have been. The person who built it made a thousand choices and got a thousand breaks you cannot see from here. The road not taken always looks smoother because you never had to walk its potholes. You cannot rerun that decision, so let the question shift from "what did I lose" to "what does this teach me about what I want now." The pull you feel toward that path is information. Find one small way to act on it in the life you actually have — a project, a side effort, a real step. The regret loses its grip the moment you start moving.