A Christmas card is a special way to express your love and appreciation for your girlfriend this holiday season. You could start by wishing her a Merry Christmas and expressing how much she means to you. You could then go on to share some special memories or moments you've shared together, and tell her how much you cherish those times. You could also include a message of hope and positivity for the new year ahead, and let her know that you are looking forward to making many more happy memories together. Finally, you could sign off with a heart-felt message of love and affection.