Not knowing what words to write in a card

Recently I bought a get-well card to mail to my sister-in-law. I had no problem purchasing the card but the problem started when I wanted to write something personal inside the card. Unfortunately, the words just weren't there! Although I liked the words that were printed in the card, I wanted to add my own words. I know that this is something that I used to be able to do very easily, but now it is not. Sometimes it is possible for me to write a special message in a card and other times like today it is impossible! I feel bad that I was unable to do this, but it is not my fault! I can't change that and those who are close to me just need to understand what I'm dealing with. I can only do my best.