Depending on how you define success, your accomplishment may be a breakthrough you had with a patient. It could also be that you facilitated a discussion in a support group that helped several people come to important realizations.
Give an example that shows you care about your relationships with your patients and their success. When they accomplish something, that is a win for you!
"My greatest accomplishment is being able to relate to patients on a personal level. I've been where they are and, because of this, they are able to open up to me on a more intense level."
"My greatest accomplishment has been finishing up my Master's degree in Counseling this past year. I am thrilled with this accomplishment and very eager to get starting in my new counseling career."
"Every patient that I have helped has been a success for me, along the way. Looking at this, and the bigger picture, I would say that my greatest work accomplishment is the positive reputation that I have in this industry."