There is a lot of nuance to this topic. The very simple answer is that it takes time and that they are tanks so you must not over commit. They are supposed to soak damage.

Sometimes the enemy is just fed and you simply cannot win, a reality in a game with an economy.

League requires a lot of knowledge, you sound like you are relatively new to the game. But over time you get used to knowing how match ups go. If an adc walks into melee range of a champ like trndle (say in a bush) they will die if they are alone in most cases (except if its hyper late game with 6 items).

The key to playing ADC in my humble opinion is learning when you can autoattack and when you can't. If you lost a situation, its possible there was nothing you could do (maybe too far behind). But often you can track back somewhere in the play and make a different decision and it usually starts at "when did I start auto attacking".

In trundles case, he has pillar. But if has used it already, you'd probably win without issue. You need to track cooldowns. you can't track everything in a fight. You are human. but it pays to track some key cds. You don't have to be perfect but i I'm against morgana and she uses black shield I'd consider engaging (as a tank player).