Adc-ing tips for when bruisers fed

I am just trying to learn how to adc and stuff and it's be SUPER HARD. Going from wukong/ekko to adc it's a huge difference. I can deal so much dmg in short amount of time or get out of a sticky situation quickly with these 2 or sneak my way to back line etc but not with an ADC.

I've watched couple videos and I understand different phases I can take as ADC in team fight where I am either hitting tank or going back lane etc but what I don't understand is what can I do vs fed bruisers?

Let's say I am vayne and I have decent farm and a couple items in my pocket and there is a team fight starting. Maybe we are at a disatvantage in numbers. Say their hecarim is super fed and I got QSS for him.

He ults in I QSS quick, I ult q to go invis, he knocks comeone else, it's time for me to go in, i get in position and I stun him to wall (this actually happened) then I hit his butt over and over but the guy still has 2k health remainin while I have 400 remainig because when he ulted he used q and that took so much out of me since you know... i am a freaking ADC with negative armor. Once he was out of stun he throws another q and i am dead.

What do I do in this situation when their bruisers are so fed and I QSS their initiation or DONT GET CAUGHT but they still deal more dmg than me because they are fed. (olaf, hecarim, vi,...)

I really love adcing thats why I want to play it but it pains me so much that I cant play it because I feel like it's dependent on the team peel and I hate that since you know... it's solo q and low gold elo................. I cri so much