You never told your ex you loved her how can you show that you care a lot for her on her birthday and get her back?
The only thing you can do at this point is try and speak with
her, get her a gift of something she would like and see how she
responds. I'd say that if you are really in love with this girl and
in the past you have been a jerk about telling her and showing how
much you loved her, I'd get down on one kneee, tell her that above
all you really do love her, can she forgive you for being such an
ass, and can the two of you start again.