Mood change. What action should i take.

It's been more then 4 years I have been going through frequent mood changes sometimes without no reason i thrown into sadness. Currently there is stress of career. I'm not able to take any decision. Sometimes it's triggers because of external situations but even I know that externally everything is fine then also I couldn't feel good again. I think It was started when I was in my hostel because of constant pressure by my mates making fun of me. Right now I am loosing intrest in everything nothing excites me. I'm feeling like I'm in trap. My physical health is also dropping down because of this stress. I don't want to see people and talk to them. Even professionally its creating to much problem. I don't know what to do, what action should I take.