Help I’m getting destroyed when I don’t play my main

Sooo, there it is. I know I’m not good at the game. I peaked platinum once playing Dr. Mundo but I found it boring since it was so passive gameplay. Right now, I play exclusively and re-grinding the ladder with Mordekaiser, from which I do really good KD/As and win alot of my games. The thing is, I don’t intend to otp this champion, I actually want to find myself another champion that I could play with (not some braindead champ like a mundo or a garen). The issue is that I FCKIN SUCK with other champs that I tried to play, such as Olaf and Darius (because they’re bruisers with sustain and damage and can splitpush AND teamfight which is what I like). And I’m talking about scores like 2-12, 1-11. So I’m kind of discouraged as I’m used to be 13-2 or similar scores with Mordekaiser so it’s just so frustrating. Can anyone help? Should I keep focusing on Mordekaiser and just tank the loss when he’s pick or ban? Should I really invest in a champion that I suck greatly in? Should I find a broken meta champion? It all feels like a lose-lose situation. Any tips and criticism is welcomed and I am open for feedback even if it’s harsh!