I don't understand the math behind MMR Calculations!

Before summer started I had a 1300+ mmr according to op.gg. Then all those stupid children got on, trolled games, had an afk in 6/10 games on average and just totally screwed me! So I quit ranked for almost all of summer. I hardly ranked. I started up again when the children were back in school. Shockingly enough, toxicity in matches dropped significantly and afk is around 1/15 games now. Now here is my question. I got railroaded extremely hard to where I went from 1300+ mmr to 800ish. I went to silver 5 promos twice now. Currently if I win my next game I'll be in promos for the 3rd time. So why is my mmr going DOWN the MORE I win??@?!? I don't get it. It should be going up. I have been the carry for my team in almost every game I have won. I contribute to my team, I get S and S- almost every time I win. As of right now in the past 20 games In solo queue I have a 70% win rate! So why is my mmr declining? it's throwing me in with people that are getting harder and harder to carry. This is driving me nuts. I recently duo'd with a friend of mine who is diamond. He jumped on his smurf that is currently unranked and his smurf was gold level so it matched us with no less than silver 3 mostly silver 1-2. between me and him we carried the team and got our adc and mid lane fed. I do just fine playing where I feel I should be. What is crazy is how if I lose a match I lose about 23 points and if I win a match I only win about 15-17 depending. So I'm not complaining about the climb here. What I'm trying to figure out is why Am I still getting thrown into matches with people who have little to no knowledge of how to play their lanes and why am I always the one who has to do all the damn work to get my team ahead still. My mmr should have gone back to at least the average of silver and it pisses me off it's lower even though i'm winning more. By the way I only duo'd two games with my friend rest were solo. Can anyone here tell me what is going on?!? How can I fix this? Or is this going to be a lost cause case till the season resets? From how it looks i'll never fix it. The queue refuses to even throw me into a match with another team that isn't primarily bronze on both teams. I'm so pissed off.