As a Riven and as an Aatrox now I've gotten destroyed. I don't know how to beat him, he slows me and then Q's me and that's all he does till eventually I can't go close and farm anymore.
Maybe I just suck, yeah. So give me advice. Thanks.
As 
you have no excuse for getting caught in his Q. You have enough dashes to get out of it, or you can knock him up or stun him mid Q to cancel it.
Sion is very vulnerable when he is using his Q. Abuse that.
first off - change your masteries for riven. secondly your runes might need adjusting. third why are you buying the hydra for gods sake on riven? because of the lifesteal? that thing is SOOOO expensive and you dont get enough benefit out of it because you're a burst attacker. honestly you're better off getting deaths dance. you need cooldown reduction and movement to go with your physical damage and hp. sion's killing you because you spend too much time buying that hydra and never get any armor/cooldown reduction so he's 1 shotting you with his Q because you cant do anything and you cant move fast enough to get away.
i got a sion skin by hextech and i started playing him...i can say u have no excuse for the q hitting u as both of those champs...aatrox has a passive as sions but sion is tankier for his w...if u max e he will do the same and build some ap-tank items if he want dmg...i main him after some other top laners but his nice :) try playing him if u cant beat him...i think i had one game sion-riven top but they surrender i think so its not in match history :(...at all sion is broken
sion isnt tanky earlygame, goes oom pretty fast. Dont take thunderlords in that matchups as riven. Take either fervor or grasp in tanky matchups. https://www.reddit.com/r/summonerschool/comments/4c9sob/super_detailed_66_riven_build_guide_items_runes/ check this guide out, its really useful. Avoid rushing things like youmus against tanks, its an all in item and you cant all in tanks from 100-0 with sunfire in his current state. Save either your w or your 3. q to knockup sion. Good luck.
Be ready to E at any time. A lot of sions will max E for the poke, and it still hurts even after the nerf. Most of the times that I won against Riven was because she didn't shield my E poke in time and let me whittle her down.
