Platinum range MMR is called "Troll City". And with good reason.
From p5-d5 you will experience having every game trolled in every possible way, almost always deliberately, for the sole purpose of frustrating players who are actually attempting to climb.
You aren't delusional. You aren't going crazy. Players in platinum play worse than in any other ELO range and its done on purpose because the majority of that range is highly egotistical about still being in low elo while simultaneously thinking no one they play with deserves to be ranked any higher. So they play worse, think worse, act worse, pick and ban worse.
I usually need about 150-200 games to take an account from unranked to diamond. I spend about 120 of those games going from plat 5-d5.
For example:
On oOBestEveNaOo this season I managed this in my placement matches and my way through gold 3-1:
https://i.gyazo.com/08bea8a0a81b7efb7df3dc6f524ed733.png
I did not earn a skip to plat because I had managed to lose 1 game in my last 4 from gold 1 0 LP to gold 1 100 LP (rip win streak skip). At this point I had a peak MMR of 2125 (roughly high d5, low d4 level.
I then proceed to get trolled in the next 100 some odd games straight, sometimes (more often than not actually, just bad luck) I had the troll on my team, some times it was on the opposing team. MY performance didn't change much in the slightest. But in the end I went from that w/l ratio to this:
https://i.gyazo.com/7ef69628a82580904319a636ab10f2d8.png
Having to play through hundreds of games with things like:
Intentional feeders
https://i.gyazo.com/219e4f31f11552062df24e2f30d5cc6f.png
https://i.gyazo.com/af4c54f8a2c24aaf711062a555ccb260.png
https://i.gyazo.com/68990e93df01cbfd7c28985233fca467.png
Challenger players boosting an account
https://i.gyazo.com/b6859a3512c6c924c124be0c57ddf6a0.jpg
https://i.gyazo.com/b6013a81b059ccdf935a5d4619a93367.png
https://i.gyazo.com/0dbe5486a5eb63ede5fd43967b8d8ea6.png
(Notice the summoner spell position swap. Only shared accounts ever feature this, and most booster are simply intelligent enough to just remap the d key to the f key and vice versa, this one is not)
Trolls
https://i.gyazo.com/0ea3a416d73c33f69963e0444447ac44.png
https://i.gyazo.com/thumb/116_w/_d6d96b2fb7a15de5cc59d6b719d1d8a3-png.jpg
And countless Teemo picks by people who simply don't care enough to actually even attempt to win a game. These are just a handful of examples from days where I was mad enough to screen shot and send in tickets to Riot support. Rest assured, every gaming session on the climb up was filled with games just like these. For about 100 or more straight games.
The problem lies in the fact that Riot does absolutely NOTHING about players who troll and intentionally ruin games in the fashion the players from these examples do, they instead choose to ban and punish the players who mention being upset at these types of trolls in chat. I know, that makes no sense. They are allowing the cause of all this toxicity to fester and punishing the victims of the crimes while giving the criminals a shiny medal, I know, i know, we all know. But the reality of the situation is this mate: Trolls and such can ONLY hold a talented player back for a period of time. Not indefinitely. No matter what, I am simply so much more talented as high diamond level player than these tools that I am assured to hit diamond if I can ramp up the sample size of games to an appropriate level that negates all the bad luck from being trolled so many times. The game is a grind for this reason. You kind of just have to accept it, learn to recognize trolls in lobby if possible so you can dodge them, and then spam games until the bad luck stops happening long enough for your skill to actually matter.