Well, I think you have to judge yourself on 3 main things. What kind of reaction time do you have, what kind of reaction time does your COMPUTER have (lol). And how well are you trading in lane.
Riven is one of those Champs I have difficulty on too, mainly because I lag a lot in teamfights and cant use Valor properly to shield damage. With any champion your using, you have to know the opponent in order to gain an advantage. Using a shield or stun at the right moment is a essential key to winning with her. Also, since her shield scales from attack damage, try not to build items that are too tanky and lean more towards damage since she benifits offensive and defensively from it.
Hope this helps a bit. Also, if you like Champion concepts, please feel free to read a concept I created and let me know what you think.
Aayran, The Telekinetic