Huge difference in my performance between Normal Games and ranked games

I use the LOL SUMO app to measure how well I'm doing in games. Usually, in Normal games it says I'm playing between Plat and Diamond level but when I go to rankeds, I usually get destroyed and it says I'm playing at Silver-Gold level (which I agree). I know people in Normals aren't playing as seriously, but I'd like not to have such a difference in my games. How am I supposed to train to get better in Normals if there's such a huge differene. I don't mind loosing a Normal, as long as I can learn something from it so I don't have to loose LP in Rankeds.