I would like to help you with this questions, but bro, you can't give your girlfriend something that has been written before. You gotta say stuff that comes from yo heart, and shows how you really feel about her. You can say what about her makes you happy, or compliment her on her looks if she's hot. Just think hard man, this happened to me, so i took this advice and everything was good.
go to one of those photo booths and take a bunch of pictures of you blowing kisses. Put the photos in with the card and then write: Pictures is all I can think of...because you leave me speechless