Question about matchmaking?

I hope this is in the right section.

I started pretty recently, last season, and I've been maining jungle. In the beginning I found it pretty easy to win games and would often carry and I climbed pretty high (for me I think) to plat 4. That was during this preseason. Then, out of nowhere I've been going on crazy loss streaks. My mmr is incredibly low and I'm sitting at plat 5 just wanting to be demoted already so I can have normal games.

My question is, why the sudden change in game enjoyment? In the beginning when it first started I tried to not change anything I was doing but it never got better, so I've started to experiment with different champs in different comps. It's either I'm getting much worse or my teammates are. It almost seems like my teammates will consistently make more mistakes than the enemy team will. Does anyone else ever feel like this, can this be explained in any way? Is this the nature of the game and this is is just my proper elo since it feels this way? Thank you for any answers/opinions. It's not that fun at the moment and I'm confused.

P.S. most of the people who are ever on my team are super tilted/toxic and give up as soon as anything goes wrong and then proceed to troll or just not try at all. I don't understand what is going on.