Maybe you need to talk aloud to yourself everything you see. Explain what is happening/happened, how did it happen, what could you have done differently to prevent it from happening again. Is there a pattern to my juking or the enemy's juking (same direction every time?) Basically am I/they being predictable. If I walk up for this CS, can they harass me with poke or all-in me? What are my summs, their summs? Abuse their cds, they can't trade after using an important ability (their man dmg ability they are maxing first, or perhaps a CC ability like blitz hook).
For item builds, you need to understand what role your character is and its good to know essentially what each item's purpose is. You also need to look at the enemy team composition and the current state of the game. Are you getting blown up by assassins? Maybe should buy a
/
. Do they primarily deal magic damage? Build an MR item. When I say "current state of the game", look and see who is most fed. There may be 4 AP on the enemy team but if the ADC is the one thats ahead, build an armor item. But usually you want to build vs your lane opponent first. Going defensive is usually best when you're behind. You may be losing damage, but that OK, that's not as important as staying alive so you can farm up for those items later. Your presence on the map is a big deal so keeping yourself alive (minimizing deaths) should be a priority. You don't need kills to be relevant; kills just help you hit your item powerspikes faster. A kill = about 15 minions. If your opponent is 2/0 but you're up 30cs, congrats, you're on equal ground with them.
It will come with time/practice. Just read what each item does and try them out. Open up the shop and hover your cursor over the item, it should tell you all its stats and passives/active. Building recommended items is OK at first but eventually you'll have to adapt & change up your builds. Just ask yourself "What do I need right now and which item will fulfill that need?". Just kinda always be thinking about which item you're going to buy next, so you don't waste any time as the fountain trying to decide (also helps to learn which components build into Legendary items and how much each cost, combine cost, etc.). Usually backing on 800-1200g will get you a decent upgrade.
Sorry for the wall of text, I hope this helps!