I think you need to hear your self. It will put in prospective what you sound like in game to others. This self realization is quite helpful. Set up windows to record your mic on game and then listen to your self afterwards. How you sound may surprise you.
Focus on what you CAN do. Be it winign yout lane, roaming, or improving your own in game. Try to find that THING that you can take out of the game, so you do not feel it was only a loss.
Just stop talking to your friends for the match. Mute the mic and you can say what you want or to keep for saying something to your friends. The silence can not be worse than a negative attitude. Yes its not as fun, but i have the feeling your not having fun at that point anyways.