I agree with Karen. A journal or pen would be nice. Also, maybe you could find a nice plaque/wall hanging with an encouraging saying on it. The other option is to make a donation in her honor to a charitable organization. If there was something in particular that you feel she helped you with, you could donate a small amount to that cause in her name (for example, to a suicide prevention agency, a group that educates people about eating disorders, etc. -- sorry I don't remember what she was helping you with, but I'm sure you get the idea). A lot of organizations will send a nice card for you to give her saying that you donated in her name, but if they don't you can always print something up yourself on the computer saying where you donated to. I used to teach high school & remember that those were my favorite gifts to receive from students b/c it showed me that they really knew what was important to me as well as demonstrated how much they had grown over the year(s).
Good luck! I'm sure whatever you pick will be truly appreciated!