It's hard to learn to say no. When people ask me for something like that, I tell them I have about 2 years of work to do first. If they're still interested, they can come back then. You can make it nice by saying you're so sorry.
At a certain age, we don't know if we have two years left, but we know we want to make the ideas dancing around in our heads. Certainly they should come first. You only need to repeat "I have about two years work to do first" one more time than she says whatever. Repetition is a technique I read about years ago in a book called "When I Say No I Feel Guilty" by Manuel Smith. It's helped me ever since.
And your work is beautiful! No wonder she asked you and I hope you'll do more beautiful work on your own things. Good luck!