Is it fact or low-seld esteem?

Lately, I've been depressed. I would always turned to the pessimistic side of me. But sometimes, I really had this negative thinking inside of me. I think i'm such a loser and there's nothing i'm good at. I know, this is not good. Everyone said that u must be optimistic. But I tried, I was optimistic at first but then, i put my hopes up too high that makes me lost everything.
I know deep down inside that I have confidence in me but the reality have shown to me that I'm nothing.
For example, my academic/studies were not that bad, but at the same time I'm trying to balance myself by trying to be good at sports too. There only one sport i think that I'm good and would have the chance to excel which is voleyball. I really have a high interest in that particular sport. But u know, just having the interest is not enough, u have to train hard to be really super good at it. U know what? I've tried my best. I trained, came to every practice. But I'm still not in that level yet. I know it takes time and lots of effort. But one thing I know that I really have commitments. I really want it soo bad. These words from the coach, would let u down. But it's actually for your own good. It pushes you to work harder and be better.
That's just one of the example.
There's more actually things that I think I suck at. I don't know, but today, I think I'm such a loser. There's nothing I'm good at.