Haha, keeping calm may be an issue of mine for sure. I have anxiety, depression and all that fun stuff, so my mood tends to be pretty volatile to say the least. I have noticed though, that when I buy a new champ and I'm excited about it, or if I'm in a good mood in general, I play far, far better. Like, my first game with Heimer, it was like 3 AM and I was super tired, but I was laughing and smiling the whole game, and went like 20+/5ish and absolutely carried through my lane. My first game Ekko I went 14/8, my second game I went 17/2 -- same deal, I was all happy and excited to play the new champ. So, keeping calm and all that may actually have some merit! I'll have to look into ways to help my mood, 'cause I do get pretty angry at the game when someone like LB, Kat, Riven and so on just instantly deletes with minimal effort thanks to their kit being so simplistic and easy to use yet borderline OP (Or absolutely OP, depending on who you talk to).
Thing is, playing just one role, I'd get bored rather quickly. So, I have a handful of champions that I know really, really well that can fill every position. For mid lane, my go-to champs are Ahri, Azir, Ekko, Heimer and TF (sometimes Nidalee, but not often). For top, I use Ekko, Azir, Fiora, Garen, Heimer and Jayce. For jungle, I use Yi, Nidalee and Tahm. For ADC, I use pretty much just Lucian (he's the only ADC I can tolerate playing), but every now and then I go Jayce. For supp, I use Tahm, Annie, Lux and Soraka. I own a lot more champs, but those are the ones I play best and have the most experience on. So I feel like I can perform adequately in lane unless I'm against some broken champ that I can't even touch.
I mean, the only reason I play League is because it's fun. :P But I do get stressed out about it sometimes, which I admittedly do need to work on.