I second surf's comment to keep trying. But I don't think that approaching random girls when you're alone at a pub is the way to go.
Being alone and approaching women you don't know is probably the least successful way to find a girl. With the exception of organised singles events, you need to take a mate along. Or stick to social events where you know a few people.
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.