nice done.. for a birthday card and love to your gf it is kinda boring, i'm not saying that your drawing is ugly. The pic is missing colours and somehow i think the hearts looks sad It looks kind of a mess, and i dont really think it fits to a birthday card.

Thats just what I think, maybe i'm wrong...

i really do love your way to draw things though, put some colours on it and the happyness will flow