Let me just tell you where you are wrong. The very fact that you don't think you can improve is the biggest problem here, closely followed by your attitude. First off, mute all if you are doing bad. Second off, the fact that you don't think you can get better by playing the game shows that you aren't trying to improve. If you are trying to improve, then norms are the best way to learn without risking your rank. Change your mentality from "i won't get any better" to "If I work really hard at it, I can get better." The second most important thing is attitude. Flaming doesn't help, and going into a game thinking you will lose is suicide. You need to go into every game with the mentality that you WILL play your best, and you WILL hold on until late.
Also, if you aren't getting any better by losing in ranked, you are doing it wrong. You should be conscious of your mistakes when you make them, and learn from them. Ranked isn't about winning games, its about learning in a competitive environment. That being said, you can still learn from norms games, however people are more likely to be serious players in ranked. But yeah, learn from your mistakes, stick to one-two champions, don't flame and keep a positive attitude. Or you can watch a stream to improve, either way, its about learning.
EDIT RESPONSE: Id suggest learning a champion in another role just so that your experience doesn't get streamlined into only one role. Id recommend Malphite top, Annie mid, or Ashe adc. Playing them will build the foundation for any champion in that role. From there you already have the skills for many other champions in those roles so learning more will not be as hard.
Keep playing soraka support too, I don't think i was clear in what I meant to say so I'm going back and adding this.
ALSO, If you aren't having fun, stop playing for a while. You won't learn to better yourself if you are on tilt.
Good luck :3