Two options: Grind some normal games to bring up your normals mmr (it sounds like maybe you improved playing ranked and now your ranked mmr is so much higher than your normals mmr that you can play anything and do well), or practice in ranked and just toss stuff out quickly if it doesn't fit your play style. Personally, I can jump on most tanky utility champs and do okay w/ them after at most 3-5 games, but I'm at least a tier down from my current rank when it comes to assassin skill bc I don't know how to play them in general as well as I do tanks and stuff. Maybe it takes more games w/ mechanically difficult champs bc it isn't just about learning their power spikes and playing the role well in team fights, bu that would be my suggestion based on what you said.