Hardstuck Plat

HI guys. So I originally started playing lol at season 3 then went off at season 5... I started again playing everyday this June 2019 from Silver and got to plat in September quite easy. But ever since I'm just stuck here. I go from plat 4 to 1 then drop again and get shitty teammates in my promos and afterwards. How can I step it up to get to Diamond?
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