What to do to fix MMR?

My account was gold 1 last season, but dropped to silver from decay, so when i placed this season i got put into silver 2. I carried another account (my step brothers) from silver 4 to platinum 2 with a diamond mmr last season, so i figured id get my main acc to diamond this season. Anyways i started the grind and within 5 days went from silver 3 to platinum 4 like 90 points with about an 80% win rate. I was getting huge mmr boosting games that were against mid diamonds, and i had a few bad games and lost those (like 3 in a row). took a break for a day and came back and first 3 games i played i had my duo's DC (yes 3 games in a row 3v5). Now i wasn't that upset but what i think has happened because of that has fucked me and I dont know what to do. I was getting teams far worse than my enemy team (Because my win rate was still so high), but the overall mmr of the game went down, so instead of me with other mid plats vs low/mid diamonds, it was me and high golds vs low diamonds. Now i did fine against the diamonds, but with gold 1s in solo and top lane (I mained adc all the way there) they got stomped and it became a "carry hard or win" fest every game for about 50 games. I obviously won some, but my win rate instantly turned from 80% to about 30-35%. I figuered the game thought i got lucky getting up, and is tanking me which would be fine but after awhile it still gives me ass teams and good enemy teams. It's not that i "couldn't" carry the games I currently have (mostly silvers for me and golds for the enemy) its just i feel like if i start winning again ill just have even HARDER teams once i get back to a platinum mmr. I feel this way because ive held a 30% win rate for 50-70 games yet my enemy teams are still an entire league above my team, so once i get to platinum ill be facing mid/high diamonds with low plats on my team. What can i do to fix this? Ive been messing around in ranked trying to get demoted to gold to try and start the grind back over but even with a 1400 mmr (yes I tanked 500 mmr) i still dont get demoted. I think the cause for this might be that the overall mmr of the game is low, but the game sees im platinum and says "lets put him with silvers vs golds which he should be able to win because hes platinum". What i would like to happen is get to a point of even teams then start carrying from there, but idk if i should keep trying to get demoted to gold or lower, or just start winning more games. Anyone with a good idea of how the mmr system works and know the best course of action let me know!