But you didn't like her then? Why regret a moment in which you actually acted upon your feelings?
If you had said yes, you would have been with a woman who you consider only a 4 or 5, that you didn't have physical chemistry with, or struggled to find.
I think you miss having her like you, that you want to be liked irrespective of pride, and that you think that if you had gone for her that you two would be together now and so you wouldn't be feeling this emotion of missing being liked.
Unless she has really shown to you just how good she is and you hadn't seen sides to her before that really lifted her value to you?