AD carries generally peak when they grab their third big item because at this point they will have attack damage, critical strike chance, attack speed, and armor penetration. That being said, I can understand feeling helpless against a fed top lane bruiser, but generally speaking, if you have a minimum of three items and a competent team in front of you, you should be fine. Also, AD carry is probably the hardest role to play as far as team fighting and raw mechanical skill is concerned. You are squishy which means positioning and micro-management of your champion is incredibly important. I come from a Starcraft II background, so small mechanical adjustments of champions came naturally to me, but it never ceases to amaze me just how many people don't know how to kite/orb walk/attack move/micro manage their champions. I would suggest watching some CLG Doublelift, if you are serious about improving as an ADC, because he is generally known for his mechanical skill and positioning.
That link is a decent showcase of his skills.
Just to clarify, when i mention orb walking or micro management, I'm referring to the minimization of downtime between auto attacks. For example, if you just right click a champion who is running away your damage will drop because your champion will attempt to move when he or she should be attacking, rather than if you moved between every auto attack. The same premise applies while kiting or repositioning your champion. If your auto attack is on "cooldown" there is no reason you should not be repositioning your champion. Obviously, the faster your attack speed, the more difficult this task becomes. This probably wasn't what you were looking for, but maybe it helped someone.