Could an actual Rioter explain the Matchmaking System?

This is what it seems like happens, particularly in Ranked: Win 2-4 games, MMR goes up. Difficulty of enemy team remains constant, quality of team degrades. (This is apparent because I continue to do as well against the enemy team as before, but my team, or most of it, feeds early and doesn't know how to recover.) So, I lose that game. Next game, same thing, only my MMR has gone down slightly, so the quality of my team is even less. Can't carry these teams for various reasons (usually, it is because all roles think they're damage carries, even when playing tanks, or if you're a tank they think every engage is too risky and never commit). Eventually, you've lost 3-5 games in a row, and the attitudes of your teammates degrade with each game. Take a break, get unsalty from the level of toxicity. Come back 1-2 days later. Lose again, same reasons. This continues until finally, you win by some miracle (usually by getting really pissed off at your team, you've always got that one toxic player berating you for mistakes but they're 0/11/2 and starts calling for surrender at 5 minutes rather than attempting to win). Cycle repeats.

Bear in mind, this isn't just complaining. I would seriously like to know exactly how the system judges who you should be up against and who should be on your team, and why it seems nine games out of ten that I'm trying lead bronze against gold.