From my experience just play games. I was stuck in high silver since season 3 so I made a fresh account to play only riven on. I got placed s2 and had trouble getting to g5. It took my about 50 games with a 40% wr (similarly to other people in this thread). That isn't including the 150 or so games I played getting that account to 30 on only riven. Basically my first 200 games I was still shit at the champion.

Now, I'm about 200 ranked games games in sitting at plat 4 and still climbing pretty quickly. Based on your post you say that you have a lot of fun playing the champ and imo that is the most important part. I think the only reason I'm climbing this fast is because of the drive to improve.

I've watched countless vods of viper, boxbox, adrian,dekar, etc. Lose your lane one game? Watch a vod of how to play it. Go even in lane one game? Watch a vod of how to stomp that lane.

Some smaller tips:

Don't be aggressive unless you know the exact damage output of both your champ and the enemy champ and you know you will win the trade.

If you don't know where the jungler is, ward up or gtfo. Even if it's warded, if you're playing vs someone like hec or nocturn post-6, respect the jungler.

Learn what to do in teamfights. If you have an adc that is 15-2 and you're going even, just build titanic and steraks and just peel. Riven has so much cc on a low cooldown that she can just be a stun bot and win the game. Don't try and carrylord every game when it's not necessary. Sorry for the wall of text but hope it helps haha.

TL;DR - If you have the drive to climb you'll climb.