Advice for mid to late game.

Long story short - I'm in a funk and I'm playing like crap. Main reason is I lost a bit of hope. I've been in and out of B4, going back to B5 never bothered me because I usually get right back into a series and back into B4, except for this last time. My game has just sank. I haven't carried a game in weeks and when I do get ahead I seem to blow it mid or late game. I'm back to B5 with 0 LP.

It's easy for me to understand what went wrong in laning phase. Usually I come out ahead during laning. My main problem is mid and late game where I don't know what I should be doing.

Any former low elo climbers have any big tips to share about the middle stages of the game? I main adc, but I also can mid with Ahri/Syndra, top as tank Sion, sup with Morg/Janna/Leona/Allistar, and jungle with Rammus (and lately Jarvan IV). I was hoping maybe someone has gone through similiar funks (even if it's at higher elo) and has some wisdom to share, be it a game tip or just words of encouragement.