Looking for Advice

I'm currently in Plat V. Now I started off the season in Gold IV and easily worked my way up from there. When I hit Plat 3, I was having issues. I quickly made my way back down to Plat V in a matter of a few days. I'm wondering if anyone can help me out and give me some pointers, help me out with what I need to work on, or even just discuss strategies to work my way to diamond. My goal is to not only make diamond by the end of the season but to also be able to play like one. (I guess you can say I'd like a free coach)

I main mid and usually adc when that's taken. Support is my third best and I don't touch top or jungle. Champions: Kat, Ahri, Annie, Jinxie, Ashe, Cait.

                                                                                                DIAMOND, THE DREAM!