I played only silly joke builds last season and dropped to silver 3. This season I'm playing a bit more seriously (no matches with builds I've never tried, outside of game I misclick and select the wrong gear template to go the correct build template before a match)
Now, I'm sitting about to enter plat, like when I try hard. I figure I can make it into plat again if I don't mess around too much.
Moral of the story. Good players get to higher rating and bad players stay in lower rating. If you're not getting higher, it's cause you're not good enough to carry through the bad games of your bracket yet.
There's a reason when I'm struggling up a bracket, at least 60% of my games are as close as 50 points by the end. Close games you barely wins, by pulling your team across the finish line is how you rank up.
If your skills have gotten better as a solo player, now you need to get good enough to make up for the 50% bad players the MMR system puts you with the balance the match. You need to overcome that obstacle, like everyone in plat+.
You could always team up, but I've never done it for a season and I've still made plat many time. Although usually plat 1-2 only. And there are players better than me.
I'm sure with more time, you'll find some builds that you can use to either tilt the enemy team. (Far point trolls). Support your team through team fights (support). Or play a build that can allow you to quickly tear apart the enemy rotations. (Holo is best for that right now)
You need to do more than play good to win at this game. You need to give your team an edge and put the enemy team on the backfoot, while staying alive and capturing points.
If you're just running mid, and your team gets wiped and this repeats over and over until the end. You probably belong in your bracket. But if you find ways to reduce the loss one game, eventually you'll turn some very close games and that's how you shift the 50/50 ratio. To reach higher brackets. You have to barely win the games you were meant to lose.
Good luck!