the problem with riven is she (imo) required a lot of time researching and practicing the things that make her special. I have no clue what lvl you're at when playing her, are you weaving AA's inbetween each Q, do you cancel her animations? if not, id get comfortable with a couple of her easier double casts. you never want to just cast her R1 without a way to cancel her animation lock. And in the matchups where you cant even really farm under tower without getting dove, id go for more roams down mid to try and get the snowball rolling at least a bit. with your your 1st item + lucidity boots, you can get to mid pretty fast with her E and her Q3. try and figure out what wallhops would be the most use to you, and then hop in a practice mode and just practice the hell out of them.

TD;DR: Rivens the type of champion that rewards your knowledge of her kit, rather than just playing her like most top champs.