Does anyone else have trouble with winning games/matchmaking?

I can go 13/2/11 as the jungler, secure 2 fire drags and an earth drag, a baron, steal enemy blue and red buffs consistently, push and get towers in groups, the enemy Lee Sin/Master Yi can go 1/10/5 and I still lose the game. I'm Gold V 54% winrate in Solo/Duo, but in Flex Queue I'm Bronze ll with a 39% winrate. Is Flex Queue the reason I'm losing or is it matchmaking? I'm sure people are gonna say "You're just bad git gud" but honestly I'm usually the one on my team performing the best. I feel like Riot needs to fix their matchmaking. Anyone agree?