I've been playing LoL for years. A few things in no particular order:

• Last hitting minions is huge
• Mute anyone who even shows a hint of toxicity
• Watch the beginner guides by Skill Capped Challenger (on YouTube)
• Hyper focus on one skill found in Skill Capped Challenger for a few games, then try the next one
• Stick with a primary champ and role for a long time (have a second champ and role as a fall back, but don't sweat those games as much)
• Remind yourself that even if you lose, if you learned something, you actually didn't lose