Anyone else read that as grinding from s3 to b1? Just me? Nevermind then.
Anyways on to the subject at hand. I am not really sure how much direct advice I can give you. I am one of those happy go lucky fellows 90% of the time. However I also don't usually have much trouble changing my mentality. For me it has always been. Am I feeling down about something? Well stop. Ok. Done.
There are a few tricks I can offer that help keep me in check at the worst of times though. One is that when you are starting to get stressed/whatever just go play an Aram. Go into it with purely the goal of having fun and just playing a silly game. Chat around with the other team and do semi dumb stuff. I am not telling you to troll. However I am saying to just play a super relaxed game. Personally I find it fun to use this site to get a random build for the champ you are playing.
Some more tricks are what others have said. Keep it firmly in mind that this is a game.
I also find that playing music can be hit or miss for some people. Turn off your LoL game music and play something fairly relaxing. Not necessarily calming music as in classical (although personally that is what I enjoy) but just a genre that doesn't overly stimulate you but that you enjoy. Pull up a video that has a couple songs in it and loop it. Playing music you own is another option. I tend to steer away from music with lyrics personally.
The music thing tends to be pretty hit or miss though. It might work for you it might not.
Finally and most importantly: If you aren't having fun stop playing (preferably not in the middle of a game) Go take a walk or something. Play destiny or Smash bros or whatever is the trend these days. Wow.... I just realized that for someone getting a degree in game programming I am super out of the loop on current trends. Huh. Interesting.
Anyways yeah. If you aren't having fun go do something else. Often getting up and moving around can help your mentality a lot. Similar tricks to this are doing jumping jacks or push ups or whatever depending on how fit you are during the loading screen. Getting your blood flowing a bit will help your thinking clarity and honestly improve your League skills at the same time.
Uhhh, yeah. I think that just about covers everything i can think of. 10 pm and I got school tomorrow so I will be heading out now. Feel free to shoot me any questions and I should get back to you sometime tomorrow. Cheers!