Today I was blamed by my mentor

I felt so sad today,because I was blamed by my mentor. I was so heartbreaking as if all my efforts that I had done were ruined thoroughly. I have mentioned here before that the staff in my office always asked me to do this and to do that.Their tasks occupied most of my time and severely distracted me from my own study. I have sacrificed so much to compensate for my missed study,but their overloaded work made me unable to accomplish my own work even though I worked until very late every day. I worked so hard,but sometimes I also felt tired, so my work efficiency is not high enough. My mentor had asked me to do something,but I failed just because of the endless work given by other staff. My mentor didn't know what I I was doing everyday,just thought I was done my own work such as studying and doing experiments. He didn't know the staff under his charge regarded me as their secretary. So today when he asked me what I had done these days,I really felt embarrassed to say. Struggling for some while,I decided to speak out the truth. He was a little shocked, he hadn't expected that his staff could let me do those many work. I felt so wronged.While my mentor's sympathy did not last long.He talked with me in a blaming tone. That feeling makes me worse as if all the work I had done is useless and meaningless. So far,my own work didn't achieve great advancement,I am aimless and overwhelmed again. What should I do? My mentor is the leader of our team, he just  has a short conversation with me occasionally when he accomplished all his work.Sincerely speaking, my current bad performance is not my own fault, being in such circumstance,what can I do?Do I have any other choice? Do I have the right to say no? The fact is I am just a student,an incapable student,what I can do is to endure because all my efforts have been taken for granted.They would not have sympathy for me even the tasks are hard,time-consuming and energy-consuming,in their eyes I am just an inexpensive labor. I don't need others's sympathy,because I know the society is so.