How to not be scared of fighting and staying on the map low on HP?

I'm maining Kata in gold and often find myself being scared to engage fights or staying in one when I'm / get low on HP. I reviewed one game where I was Vex vs Naafiri mid and I saw myself instinctively clicking away in one fight even though my jungler is coming for a gank mid. He got the kill there but we could very much have missed the free kill there

I see many montage plays where the Kata (or any other melee champ) player commits to a fight or stays even at low HP and wins and gets a quadra instead of one or a double kill or takes a fight in lane and barely lives with the kill.

I reviewed another game where I played Yasuo and was fed but I hesitated to commit flash onto the enemy Xerath support and died to his Volibear jungle in a team fight when I could have flash autoed him for an extra kill and potentially kill Volibear too considering how far ahead I was. Similar story, hesitating to commit big stuff like my flash that I also do as Kata or any other champ even though using it I could have lived.

Similarly after I win fights and I'm low, I default to backing even though there are often better plays or unless the enemy team is all dead.

How can I determine when to stay on the map in these situations where I'm low on health? How can I learn to trust my flash or triumph rune or lifesteal from stuff like BORK better?