I never saw how you play but if prople say you throw mid-match... Never do that. I did over 600 matches this season, and some were won when we were losing 575/225 - it made their team easy to kill with all these stacks and recover. Many matches were won last 10 seconds by melee LB. So don't give up till you see Defeat or Victory on screen.
Also, after losing -200 per day several times and being demoted I can recommend to STOP queing this day after 2 loses in a row, because it'll be a depressing and infuriating downward spiral and a lose streak. If you win, keep pushing your luck till you lose, then stop. Make small steps in your rating management and don't be too greedy because you can lose everything without a chance to recover if close to season end.