There's only so much you can do; I'm in a similar position as you - I've been unable to improve any further so I've been stuck in D2 for almost 2 years. It's just my ceiling I guess. We're all limited by our innate characteristics, both physical and mental. Plus there are other factors outside the game itself which limit our available time of playing etc. Furthermore, practice can only go so far. Okay, everyone can reach Diamond 5 if they play enough. Probably even Diamond 3, all you need is a decent mentality (one that at least allows you to improve) and enough practice. But at some point everyone reaches a brick wall which they cannot surpass. There's a reason why there are drastically fewer people in Diamond 2, 1, Master and especially Challenger as opposed to low Diamond and Plat. I guess we don't all have the same ability and talent, to call it that. Practice is key, but it can only go so far. People in challenger are naturally more gifted than others. People like Faker are unique specimens. Just like Ronaldo, Messi, etc. in football.