On your "My Account" page toward the bottom, you will see a list of all the cards that you have sent. Next to them are the amount of times the recipient has viewed the card and a way to view the card you sent with your message or delete that card altogether.
To view the card click the little magnifying glass. To delete it, click the red circle with the x.
Another function that is under construction is the ability to search by name the cards you have sent. So if you have sent Aunt Bertha 12 birthday cards in the last 12 years, you will know which ones.