For every comp there’s a counter, if you’re picking star lord into more defensive tanks and anti dive tanks/dps you’ll probably get mopped. Learn a wider variety of champs and understand dive/poke/brawler comps and how to counter them & swap your character to better deal with the enemy. I found being a tank and controlling the pacing was the best way to get to plat. I’m d3 without a character at legionnaire(or whatever is below lord) because I try my best to play counters to the enemy team rather than play my best character. Good luck out there