Why is mmr still fvcked up?

So I started this season in Silver II (finished Season 4 in Gold III but had bad placements, think 3-7), I got to Gold again really quick and a couple months later I reached Platinum. My winrate then was 126-103, I got up to Plat IV very soon, then the misery started. I would lose alot of games (even 12 games in a row at some point rofl) but kept playing until I got demoted to Gold I. My mmr was Gold V level at that point and I got 6 LP for a win at some point. So I needed like 15 wins just to get back to my Platinum promos since I would gain an average of 12 LP and lose 25 for a defeat, however it didn't take me long and I got back to Plat BUT MY MMR DIDN'T IMPROVE much. I still kept facing mid Gold players or other Plat Vs with bad mmr even though I won like all of my games since making it back to Plat my mmr still sucks...I am at 211 wins and 192 losses right now after getting back to Plat IV 2 days ago but even in my Plat IV promos I faced alot of Gold players as you can see in the images attached.

So yeah...why is that the case and what can I do? I mean, it leads to more wins playing against players below my own skill level but I would rather compete against people in my elo.