As A Teacher, I Feel Very Tired And Overwhelmed As

As a teacher, I feel very tired and overwhelmed as each new school year begins. I seem to only enjoy my off time in the summer when I have no stress and demands. I have taken Zoloft for 12 years. I have difficulty dealing with daily demands at work emotionally and mentally. I can't wait until the next summer. I also walk every day. What is wrong with me? I absolutely hate having to deal with all the many demands at work. Thanks.