Why am I depressed? Please help?

For as long as I can remember I have never been turly happy, there's always been something going wrong. But I've always been very good at hiding the way I feel and putting a brave face on. As I get older it become more and more difficult for me to pretend. Especially in the past couple of years. Recently I've been struggling to get up in the mornings and go to work, which I used to really enjoy but not now. I only socialise if absolutely nessasery I sleep a lot and have lost my appetite recently. I would never attempt suicide but I feel like I just don't enjoy life as much as you should and I'm just so tired of having to pretend everyday. What's the point in being here if it makes me feel so terrible all the time. I'm 21 and I can't remember the last time I was content with life. What's wrong with me? I've tried talking to people but doesn't seem to make it any easier.
Why am I depressed? Please help?
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