Mood swings. What is the problem?

I had passed SSC with 91 percentage and I went to science for 11 th then I dropped out because of depression then I went to arts but didn't attend. Then I had completed diploma in DJ and passed HSC as a private candidate with 82 percentage in arts. After that I decided to do hotel management and I am very excited about it. Now I'm doing bsc in hotel management. When I decided to do this I was very excited, happy and motivated and energetic. But sometimes I feel very uncomfortable with my decision and feels not to do anything and quit the course. Again I wanted to do and study for hotel management and I feel motivated towards and feels happy . But again feel confused, anxious and my heart beat gets fast. I feel anxious and sad. But sometimes feel happy for few minutes or hours. My cleft palate surgery had been done at age of 11 months. Now I'm taking duvanta medicine as per doctor prescription. Now what should I do and how should I overcome this problem and exactly what is the problem?