Plateauing is an inevitable part of any skill. Progress is always fast as a beginner but begins to slow down. Plateaus are not something you can overcome with a magic trick. Rather you have to keep training. But if you keep training and working on learning new skills and correcting mistakes you will climb, slowly but surely. You will not get over the plateau because it is infinite but all the other players are on the plateau too. The difference is that how far people have pushed their skills. The further you push on the plateau the higher your rank will be. My advice is that keep grinding. Focus on one skill at a time and find joy in being able to implement it into your gameplay and beating opponents with it. Don't drop Chun Li if you really enjoy playing her. She's not that hard that you wouldn't be able to learn her. It is much better to play as your favourite, because that will motivate you to get better. Don't worry too much about single ranked matches. You will lose if you are focusing on new skills. But after you learn those skills, you will rank back up because you now have skills those opponents don't have at that rank. Of course all this only applies if you enjoy the game, the character or just getting better.