A few pieces of advice that may, or may not help. 2 different topics of advice actually.
1: Remember that you arent going to win every game, even games where you play amazingly, perfect almost, you can lose because at the end of the day it is a 5v5 TEAM game. It seems like you have a pretty good head about you so I would imagine you understand that concept pretty well already. What I really want to talk about is that mindset, and overall mentality is 100% just as important as physically playing the game. I dont mean strategy or thinking about what is going on or outwitting your opponent, those are all important as well, but I am talking about morale, and the mentality that ties to it. Keeping the right mindset about the game before, during and after the game, whether it is a victory, a loss, a complete stomping win, or a dreadful annihilation of a loss. It is always important to keep the right mentality. Avoiding negativity like "this Twitch is split pushing we cant win teamfights unless he groups" and instead focusing on potential, and positives such as, "if we hold them at tower, or make them take long enough to destroy it we can give twitch enough time to take a tower in return" is really important (and sometimes incredibly hard). Also not taking games too hard, remember that you cant win them all, and you did the best you could for that particular game. Could you make improvements? Probably, Could you do things a bit definitely? Sure. Would it have won you the game? Who knows. The fact of the matter is you didnt win that one, but that doesnt mean you cant win the next one. Or the one after that. As long as you are putting in the full effort in trying to win as well as making a legitimate effort to trying to clean up mistakes (no matter how minor they are) and staying in the mindset of "what can I do in this specific game right now, that will help this specific team win this game"(sometimes that means Going 15/0 and playing full ap burst amumu and suiciding in to the enemy team to kill an enemy carry or two, Sometimes it means grabbing a supportive item like locket or redemption or even knights vow and just sitting on top of your adc, no matter where he is or what he does to make sure he stays alive, and it could even mean leaving towers to die while you split push to trade towers instead of fighting at a disadvantage, losing a few members of the team and then the tower on top of that anyway) then that game can be a personal victory even if it ended in a loss. But the important part of mentality, is DONT GET STUCK ON SOMETHING, experience it, good or bad, figure out what went right or wrong, decide what you will do if it happens again and then let it go, move on to the next teamfight/tower/dragon/baron/game/day/week/season/etc
2: Dont take this as me trying to tell you what to play or not to play, I am not an expert and dont try to pretend like I am one. Having said that I do feel like I have a pretty decent understanding of things (MAYBE even slightly above average, but who am I to decide). The Jungle as Im sure you know has the unique ability to affect essentially the ENTIRE map at pretty much all stages of the game. The jungle changes this season have ballooned the junglers ability to impact the map tremendously. Currently junglers with high efficiency jungle clears are absolutely taking over the game in an unstoppable fashion. Perhaps transferring your priority to champions that have increased early game impact (hecarim, lee sin, rek sai, rengar) as well as rediculously fast, safe and efficient jungle clear mechanics will help give you the edge in your games. Im not saying amumu is a bad pick or that you shouldnt play him, Im suggesting that the meta high tier highly contested picks are certain champions for a reason. That reason is that in an evenly skilled matchup where neither side make any mistakes these champions have an advantage just on a champion level (or like for support in janna's case, she is just so entirely easy to play that the chances of you messing up are so low that her weaknesses on a champion level are overlooked). So I suggest playing the efficiency driven, high impact junglers and force advantages that way. If you get into a situation where one of your carries (in your previous case the twitch) feels like they have something better to do than what you think is the right thing, try increasing communication. Pinging more ahead of potential fights, pinging enemy movements, pinging your movements, pinging suggestions for things you think your team should do, typing suggestions (FIRE DRAG NEED TO CONTEST, etc) and if they still dont start to get on the same page as you, change your thought process and play around them. If twitch wants to split, either distract and delay the enemy team, or join him. Making a decision to follow a 5 man mistake is more likely to have a positive effect than trying to make a split decision where you are correct but you lose it anyways because you are split up on the subject.
Overall remember, that was one game and you have the ability to win future games. Mindset, and morale are just as important as how effectively you can press your buttons or click, especially over the course of multiple games.
Hope at least part of this helps. If it doesnt I apologize for wasting your time. Good luck on the rift.
