My personal opinion is that this is caused by going into the game 'expecting' to win.
Instead, try going into the game 'hoping' to win, but 'plan' to have fun - even if you don't win. Be glad you're participating in an activity you like.
I like racquetball. It's fun even when I lose.
Why can't League be the same?
Play to win, sure, but don't ONLY play to win. Play to get better. Play to relax. Play to hang with friends (or make new ones). Or play to watch other people (see what they do that works and what doesn't).
Change your expectations, and I promise your attitude will surely follow.
Also, try setting realistic goals for yourself.
If you die all the time, try your best to play defensively - set a minute limit, such as "I'll try to stay alive for 5 minutes" or "until laning phase is over", etc.
Can't beat your lane enemy? Try for a stalemates instead.
Creep score always low? Practice that last hitting, and aim for a little higher CS than usual.
Feel every game is the same? Change it up a little. Decide you're going to try to make 'one good play' this game. Or decide you're going to focus more on map awareness or ward placement.
Bored with your champs? Try doing something a little "off meta". A little. Don't overdo it. If it works, branch out a little at a time. Playing max healing/shielding power on Lux
(
, etc) can be fun and helpful.
Whereas stacking mana items on Garen
(
) is NOT.
You can also create 'missions' for yourself. They don't HAVE to be in the little mission box. Decide that the next game, you're going to secure 2 dragons for the team. Ward. Watch. Try to coordinate and make it happen. If it doesn't, try it again next game.
And if you need a reward, then reward yourself in your own way. Finally got mastery 5 (or 6, or 7), or finally broke 300 cs (or 200, or 100)? Buy yourself a skin. Or a chocolate milkshake. You got an S+ on Alistar and were honored by your whole team? Take a break and have an alcoholic beverage (or blaze up, it that's your thing). Reward yourself with whatever motivates you.
And last but not least - above all, it is a 'game'. It isn't work (unless you're a pro). If you're not having fun then find something else to do that you enjoy. There's nothing wrong with stepping away from the game (or anything else that isn't required in life). Take an hour or a day or a month or whatever. League will most likely be here when you return.