Two things which may help, the first is are you duo que? That may mess w/ gains and losses. Second is probably the low MMR issue, unless you go on a streak for wins you don't really bring mmr up that much, especially depending on the games. Going 9-3 is great, but you lose more if your team was expected to win and gain less, so if you're matched in a bunch you're expected to win, and still lose some anyway then you won't get ass much because it's really like going 9-7 or 9-8 because you were so favored going into the games you were expected to go 10-2 or 11-1.
Last thing is about investing time and learning. This is something I did a lot of (and if I can get good enough to hit plat this season I'll write a guide about it. Went b4-g5 season 3 learning, and was b3 this season and now mid s5 after a couple weeks when I started learning again), and it is also something you can waste time if you're doing it wrong. One suggestion I'd recommend is check out How to top lane on youtube if you haven't his channel covers mechanics like wave control and freezing a minion wave that are really important in all lanes. The second is how you apply the effort you put into learning the things you talk about.
I picked up Heimer this season because he's fun (and because I can play him top and mid so I get to play him more often since 2 lanes) and started watching a heimer main's stream and really analyzing my wins and losses. Studying mechanics, focusing a lane, watching streams are all good, but it takes conscious effort to adjust behavior. For example, I did a lot of the things on heimer that I saw in the stream but I realized I wasn't auto attack harassing nearly as often as the streamer. I started focusing every game on auto attacking as many times as I could do it from levels 1-6 and focusing on doing it when they had to choose between getting creeps or hitting me, while my turrets would get the creep I would auto if I didn't attack them. I saw the CS difference between me and opponents shoot up and started winning more games in bronze because I had a lane advantage. Don't just watch streams and main a lane, focus on the specific mechanics that build advantage for what you're playing and where you're not doing the best you could. Remember the best way to get better is to assume that every loss, every death, everything that goes bad in a game is your fault. Look for a way to do it better next time and even if it wasn't your fault, then you might find something you could do better, or find a way to out play th opponent that keeps them from exploiting the mistakes of your team.
