Good for you for trying to stay positive. I too struggle with staying happy while playing LoL. Losing can be so disappointing, and by acknowledging behavior that you want to change, you have already accomplished a critical first step. This is because until you acknowledge a problem, it's not going to change!
One thing that might help is, when things in the game go bad, remember your goal of being a positive player (you'll have to work out what "being a positive player" means to you). When your teammates are flaming against each other, or your jungler didn't help you out when you needed them most (or whatever circumstances give you the most grief), remember that goal. Then, your focus changes slightly from winning the game per se to playing the game gracefully, no matter the outcome. This way, even if your team ends up losing the game, you still accomplished your goal--you tried to be a positive player and help out your team. Another thing that happens when you remember your goal for yourself is that your focus also changes from what other people are doing to what you are doing. You are the only person whose behavior you can truly control, so why put all your effort into trying to get other people to do what you want? It's madness!
Another thing that helps me personally to tone down strong emotions while playing is to listen to relaxing music while I play. I might listen to new age or upbeat religious music, because these things calm me down and make me feel content. Find what helps you feel the best.
Finally, Riot's "summoner's code" has some things written there about what kind of behavior Riot wants to see in the game, so it may be helpful to review what it has to say and see how you can be better at incorporating those principles into your play. Here's the URL: http://gameinfo.na.leagueoflegends.com/en/game-info/get-started/summoners-code/
I know there's probably great advice that I haven't given here, but this is the best I've got so far. Good luck!
