It's good to be critical of yourself, but don't take it too far.
Especially if you're suffering from depression.
Over the past... year or so, I gradually got it into my head that I was incapable of doing anything right. It got to the point where I began to think I didn't deserve to play this game anymore, and I ended up sabotaging myself by cursing out everyone I played with for no reason until I finally got banned from this game (that happened about a month and change ago). I also lost most of my friends outside the game for the same thing as well.
Since then, I've tried to just move on, but trying to just forget about this game is rather difficult when most of the friends I still have are League players, plus the fact that I got one of those gameplay surveys in my e-mail a week ago.
I actually started to try and level up a new account, but pre-30 League is barely even the same game. Between jungle/support not being properly played often, the lack of champion diversity when most games are blinds involving mirror matches of free rotation champions or 450's, and non-smurfs not really caring about objectives at all, there's not much useful to learn. It's still an arduous grind to level 30 as well, even with the halved XP to reach it, never mind all the IP you need to sink in order to actually get to the meat and potatoes of the game.
This all could have been avoided if I'd just not gotten so far down on myself as to actively sabotage any chance I had of improving, so I have nobody to blame but myself. As much as I want to blame this game's pre-30 for being a mockery of what the game is supposed to be like, that wouldn't do me any good.
To summarize, don't let doubt control your life. It'll take and take until you have nothing left.