Winning a girl's heart happens over time. It's something which comes with her getting to know you and finding out that you and she make a good match.
So the next thing you do is start get to know each other better. You don't say how well you know her, so all you can do is up the ante. If you're not dating, then ask her out. If you are already dating then take her for a weekend away somewhere nice.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.