Is there a way to bookmark affirmations or create a list of favorite affirmations?

Yes.  

How to Bookmark an Affirmation / Add it to Your Favorites:

1.  Create an account and login.  
2.  Navigate to the Affirmation
3.  You should then be able to see a buton called "Favorite This".  Click on this button
4.  You will receive a confirmation message : "This item has been added to your Favorites"

How to See Your List of Bookmarks / Items you have added to your Favorites
On the left menu, under your name, you should see several links such as "My account, "My favorites" and "log out".  Click on my favories.