If she's in a relationship, then let the topic drop. Respect that she gave you a 'no' and move on.
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.
I think she was letting you down easy, Doggey. I don't think she wants you and her to date, only ever be friends.
Alright... I suppose I'll just see how things go.