I do feel sad I always feel sad

I've been depressed for more than 4 years and I always feel sad sometimes for a reason sometimes without a reason it just happens like that , people who knows me thinks that I'm all sunshines and rainbows but in fact I'm not.. not at all I'm so pessimist and negative about myself, I see everything in dark.. i always try my best to not hurt people I always think 100 times before saying a word but do others do the same for me I think not. I've been feeling sad all the time, sadness become a part of me, or you can say it ate me as a whole , and I can do anything about it, so I always wear the happiness mask and show everyone that I'm fine when the reality is I'm not okay, I'd rather keep that sadness for myself that worry others about me