How can I say no to a friend's request to become a business partner?

Over the past several years, I developed a home craft business. Now that it is successful one of my friends wants to be involved. She sends messages asking to get together to discuss ideas for new products and expanding the business. However, I am not interested in having a partner. How can I let her know that I don't want a partner -- without coming across as mean or hurting her feelings? Also, since I want to support and encourage my friends’ interests, I’m struggling with guilt for saying “no.” How can I overcome that?