1- Aways think before take action.
It seems difficult at first, but you'll soon get the hang of it.

2-If you wanna get so much better make your own rules.
for example I have several rules,
keeping indented till 6 (focus on farming)
Aways recall when your HP < 30%
Try KDA > 2 Every game even with support

3- Focus on making just one item its wrong:
Ex:
If you have 1450 gold don't sell you dorans (sustain) for make bf earlier.
In top rush hydra as first item, when you're loosing just buy tiamat and goes tanky.

4-Don't blame your teammates, if you have this mentality:
I made mistakes and I can change for get better
You will aways get better, of course you will loose some match because of your teammates but foccus on your own mistakes.

5-Write your fails, your objectives, the throws. Whatch your own games.

Ok I hope its help.

Sorry for english, if you dont undertand anything i can make better/edit or something else